The contract for South Western trains will be the first to expire in May, followed by Essex Thameside two months later.

The Labour leader has vowed to take the different railway operations, currently franchised out to private providers, back into public ownership as they expire.

But there have been warnings from the industry that Labour has significantly underestimated the cost of renationalising the railways, especially when it comes to running costs

Two further franchises are set to expire in 2026 with the last contract, operated by West Coast, coming to an end in Autumn 2022.

Sir Keir could choose to forge ahead with his renationalisation plan or try to delay it by granting new contracts to train companies. As more and more rail franchises came into public ownership Sir Keir would likely face questions from left-wing MPs over his abandoned plans to nationalise other industries, including water and energy companies.

Keir Starmer visits Hitachi Rail to present Labour’s roadmap for 'Great British Railways'CREDIT: Ian Forsyth/Getty Images

If he failed to take action, he would be the first Prime Minister in history to tax the state pension, with the five million poorest retirees who solely rely on the handout suffering the most.

That would raise the prospect of all pensioners having to fill out self-assessment tax returns on their state handouts, with HMRC pursuing those who failed to save enough across the course of the year to cover their tax bill.

Sir Keir would therefore face a three-way choice

He could choose to mirror current Tory plans and raise the income tax threshold just for retirees, but that would spark accusations of intergenerational unfairness.

Alternatively he could raise the basic rate threshold for everybody, but to do so he would need to find billions from elsewhere. Or the third option, he could do nothing and let events take their course.

The Labour premier will inherit a country which has seen record numbers of arrivals from overseas over the past three years, during which period 1.9 million more people came to Britain than departed.

The first few years of a Labour government would see net migration fall from its record high of 764,000 last year to around 315,000 by the start of 2028, according to ONS figures, provided that Sir Keir does not reverse Rishi Sunak’s recent visa clampdown.

But that would still mean a city the size of Nottingham was moving to Britain every year, and would leave net migration well above the 256,000 it stood at in 2010, the last year of the New Labour government

The ONS predicts that after 2028 the figure would flatline at 315,000, meaning Sir Keir would face pressure to launch a new crackdown.

The average UK house price dropped to £285,000 at the end of last year but, after a further decline this year, prices are set to start rising sharply.

By 2029 the average UK house price will surge past £300,000 for the first time.

This would present something of a political dilemma for Sir Keir, who has promised younger voters that he will bring down housing costs by turbocharging homebuilding

He would have to choose whether to respond by further relaxing planning laws to massively boost supply, or with policies to make mortgages and deposits more affordable such as a new Help to Buy scheme, the likes of which have been accused of further stoking property price rises.

Spending on health is projected to have to increase to 10.1pc of national wealth by 2030, up from 8.1pc at present, as pressure on the NHS and social care grows.

Inevitably that means the Labour leader would have to decide whether to further ramp up taxes on working people to pay for the increasing cost of looking after the elderly.

By 2033 it is projected health spending will hit £278bn up from the current £182bn, leaving the Labour leader with an almost £100bn sized black hole to fill

The huge spike in spending will come about as the end of the decade marks a major demographic tipping point for the UK as, for the first time, one in 10 of the population will be over 75.

At the same time the proportion of under 25s will fall to just a quarter, as Britain becomes an increasingly ageing society.

Nowhere will that be more evident than in London, where the Black, Asian and minority ethnic (BAME) population is set to hit 45pc by the early 2030s, according to Greater London Authority projections.

At the same time high levels of net migration will continue to increase the overall proportion of the UK population which is foreign-born, a tally which currently stands at around 14pc, having risen from 8pc in 2001 and just 4pc back in 1951..

Sir Keir would be the one to oversee this revolution, which would come with huge challenges as well as opportunities.

Forbes has predicted that by this point humans will be using AI as personal assistants, tutors, counsellors, accountants and lawyers - and even in a more personal capacity as romantic partners.

All of which will pose a number of questions for the then Prime Minister, including the huge impact that robots taking over previously skilled and well-paid jobs will have on the economy and Britain’s workforce

Will he choose to try and curb this new technology with legislation protecting workers’ rights - as will doubtless be demanded by Labour backing unions - or let the technology take its course and pivot Britain into a new economic era.

Keir Starmer watches Starship delivery robotsCREDIT: Darren Staples/Getty Images

Both have been massively delayed and are coming in heavily over budget, meaning the new Prime Minister would have to choose whether to keep on pumping in billions of taxpayers’ cash or try to cut the country’s losses.

On present projections HS2, which has been reduced to a London to Birmingham line and is still set to cost north of £100bn, is finally set to come into full service in 2033.

That very same year Sizewell C, in Suffolk, is due to finally power up following years of legal wrangling which have pushed the price tag up to £26bn.

From 2035 the sale of new petrol and diesel cars, and the installation of new gas boilers, are all set to be banned to reduce carbon emissions.

It could be a tricky moment for Sir Keir to navigate with concerns that electric cars and heat pump technology may still not be up to the job.

The deadline would also pose him with an economic and security imperative to try and boost domestic production of green technology like batteries, the vast majority of which is currently manufactured in China

Failure to do so would leave the UK at the mercy of Chinese imports once the bans came into place, with enormous geopolitical consequences.

This is also the year that, on a personal note, Sir Keir would eclipse Margaret Thatcher’s 11 years in office, which is a record tenure for modern times.

According to OBR estimates it will take more than a decade in power for Sir Keir to finally register annual economic growth of above 2pc, moving Britain into the global fast lane once more.

The figures show how he faces an uphill struggle to fulfil his pledge to make Britain the fastest growing nation in the G7.

In contrast Sir Tony Blair, on whom Sir Keir has modelled his leadership, recorded a healthy growth average of 3.4pc across his first five years in office

Lower growth in the early parts of a Labour government would raise the likelihood that Sir Keir would have to ramp up taxes to pay for increased spending on public services.

If that moment arrived it would present a huge challenge for Labour, who by this point would be 13 years into government, equalling the party’s entire spell in government under New Labour.

And whether or not that symbolic crossover occurred, working with Western allies to combat an increasingly belligerent Beijing would be one of the biggest challenges of Labour’s near two-decades in power.

Whilst the Tories have commissioned new submarines to replace the current Vanguard class, which will be phased out from the early 2030s, they put off a decision on the future of the nuclear warheads they carry by extending the lifespan of the current stock of Trident missiles into the 2040s.

A Labour prime minister would therefore have to choose whether to try and extend the life of those missiles once again, with the US planning to keep its Tridents in service until the 2080s.

Alternatively they could order new nuclear missiles at a much higher cost or, more controversially, scrap the UK’s nuclear deterrent already as has previously been demanded by figures on the Labour left.

HMS Vigilant carries the UK's Trident nuclear deterrentCREDIT: James Glossop - WPA Pool/Getty Images

September 1, 2039 will mark the 100th anniversary of the outbreak of the Second World War, a huge symbolic moment for the whole European continent.

Labour’s then-leader would be the Prime Minister representing the UK at a series of events,including the centenary of the Battle of Britain a year later.

By this point it is unlikely that Sir Keir would still be in Downing Street - he would be 76 this year and have served 15 years in power.

Official forecasts by the OBR state that spending on the state pension and associated pensioner benefits is set to rise from 5.9pc of GDP at the moment to 6.8pc by the start of the 2040s.

That would see the overall cost of supporting retirees spike by almost £25 billion a year, rising from just over £150bn at present to around £175bn.

Meanwhile the combined cost of funding the state pension, pensioner benefits, the NHS and the social care system is set to hit an eye-watering 18.7pc of GDP.

The extent to which that growing cost would squeeze the money for other areas like defence, education and policing would leave Sir Keir facing a number of options, including the possibility of real-terms cuts to the state pension by scrapping the Triple Lock.

Even more radically he could look to abolish the system altogether and replace it with a newer, more affordable model.

Projections by the Health Foundation suggest that by then some 9.1 million Britons will be living with a major illness - a whole 2.5 million more than in 2019.

That will mean the number of people suffering with sickness goes up to nearly one in five of the entire adult population, which increases in mental health problems, chronic pain and diabetes accounting for much of the rise.

By this point further boosts to health spending may be becoming increasingly unsustainable, meaning the then Labour prime minister may face having to make difficult choices about the future model of the NHS and healthcare in Britain.

Sir Keir Starmer, leader of the Labour PartyCREDIT: Leon Neal/Getty Images

By this point the Tories - or another rival right-wing party - would have eaten away at Labour majorities in successive elections and would be primed to sweep to power at the election.

As we have seen with the Conservatives this year, the prospect of a spell in opposition usually leads to MPs and ministers looking outwards, towards their future careers, rather than focussing solely on the job at hand.

That means little policy work gets done, whilst it is possible that the incumbent Labour leader could be ousted in a final throw of the dice.

CREDIT: Telegraph