The British economy will suffer a £252bn hit if Theresa May carries out her threat to leave the European Union with no deal, a new analysis of official forecasts shows.

A no-deal Brexit would see GDP plunge by more than a quarter of a trillion pounds over 15 years, according to the study shared with The Independent.

Less damaging exit terms, under which Britain would secure a free trade agreement with the rest of the EU, would still result in national output being £131bn lower over the same period.

And even a Government U-turn – leaving the UK in the EU single market and customs union – would swipe £52bn from economic growth, the calculation says.

The Best for Britain Group, which is campaigning to halt Brexit, said its research – based on the Government’s own leaked analysis – fully exposed the “Brexit black hole at heart of the economy”.

“Sadly, now we are seeing the economic analysis becoming project fact and it means that we are facing a massive Brexit blow,” said Lord Mark Malloch-Brown, the organisation’s chairman.

“Losing billions means we will have less cash for the NHS, our schools and our public services. This all shows the best option for communities up and down the country is to stay in and keep the deal we have.”

The calculation is the most comprehensive attempt yet to put a price on the economic damage forecast in the Treasury Brexit analysis which the Government was – eventually – forced to release to MPs.

It predicted a no-deal Brexit, leaving Britain trading with Europe on World Trade Organisation terms, would reduce growth by 8 per cent over 15 years.

The biggest issues facing UK on leaving EU Show all 8 1 /8 The biggest issues facing UK on leaving EU The biggest issues facing UK on leaving EU Post-Brexit immigration workers sorting radishes on a production line at a farm in Norfolk. One possible post-Brexit immigration scheme could struggle to channel workers towards less attractive roles - while another may heighten the risk of labour exploitation, a new report warns. PA The biggest issues facing UK on leaving EU Customs union A key point in the negotiations remains Britain's access to, or withdrawal from, the EU customs union. Since the referendum there has been hot debate over the meaning of Brexit: would it entail a full withdrawal from the existing agreement, known as hard Brexit, or the soft version in which we would remain part of a common customs area for most goods, as Turkey does? No 10 has so far insisted that “Brexit means Brexit” and that Britain will be leaving the customs union, but may be inclined to change its position once the potential risks to the UK’s economic outlook become clearer. Alamy The biggest issues facing UK on leaving EU Northern Ireland-Irish border Though progress was made last year, there has still been no solid agreement on whether there should be a hard border between Northern Ireland and the Republic of Ireland. To ensure borderless travel on the island, the countries must be in regulatory alignment and therefore adhere to the same rules as the customs union. In December, the Conservative Party’s coalition partners, the DUP, refused a draft agreement that would place the UK/EU border in the Irish Sea due to its potential to undermine the union. May has promised that would not be the case and has suggested that a “specific solution” would need to be found. Getty The biggest issues facing UK on leaving EU Transition period Despite protests from a small number of Conservative MPs, the Government and the EU are largely in agreement that a transitional period is needed after Brexit. The talks, however, have reached an impasse. Though May has agreed that the UK will continue to contribute to the EU budget until 2021, the PM wants to be able to select which laws made during this time the UK will have to adhere to. Chief negotiator Michel Barnier has said the UK must adopt all of the laws passed during the transition, without any input from British ministers or MEPs. EPA The biggest issues facing UK on leaving EU Rights of EU citizens living the UK The Prime Minister has promised EU citizens already living in the UK the right to live and work here after Brexit, but the rights of those who arrive after Brexit day remains unclear. May insists that those who arrive during the transition period should not be allowed to stay, whereas the EU believe the cut-off point should be later. Getty The biggest issues facing UK on leaving EU Future trade agreement (with the EU) Despite this being a key issue in negotiations, the Government has yet to lay out exactly what it wants from a trade deal with the EU. Infighting within the Cabinet has prevented a solid position from being reached, with some MPs content that "no deal is better than a bad deal" while others rally behind single market access. The EU has already confirmed that access to the single market would be impossible without the UK remaining in the customs union. Getty The biggest issues facing UK on leaving EU Future trade agreements (internationally) The Government has already begun trying to woo foreign leaders into prospective trade agreements, with various high profile state visits to China, India and Canada for May, and the now infamous invitation to US President Donald Trump to visit London. However the UK cannot make trade agreements with another country while it is still a member of the EU, and the potential loss of trade with the world's major powers is a source of anxiety for the PM. The EU has said the UK cannot secure trade deals during the transition period. EPA The biggest issues facing UK on leaving EU Financial services Banks in the UK will be hit hard regardless of the Brexit outcome. The EU has refused to give British banks passporting rights to trade within the EU, dashing hopes of a special City deal. However according to new reports Germany has suggested allowing trade on the condition that the UK continues paying into the EU budget even after the transition period. Getty

Based on average GDP growth over the past 5 years, that would cut output from £2.62 trillion by 2033 to £2.36 trillion, Best for Britain calculated – a loss of £252.4bn.

This week, Theresa May’s spokesperson insisted the Prime Minister was committed to Brexit even if MPs vote down any deal she brings back, saying: “We are definitely leaving the European Union.”

Leaving with a Canada-style free trade agreement would see growth cut by 5 per cent which, the study says, which would mean a GDP loss of £130.8bn.

Meanwhile, staying inside the single market would reduce growth by 2 per cent – or GDP by £52.3bn, Best for Britain calculated.

Lord Mallach-Brown added: “This Government are not competent to deal with the challenges ahead. They are going to hit people in the pocket through their hapless handling of Brexit. Starkly, we are facing a Brexit bombshell for the economy.”

The organisation – which hit the headlines when it was revealed it is partly funded by billionaire financier George Soros – also calculated the hit to the public finances implied by the Government’s analysis.

It found tax revenue would plunge by £82.4bn over 15 years under a no-deal Brexit, by £42.7bn with a free trade agreement and by £17.1bn if there is a “soft Brexit”.

The first figure is almost identical to that set out in the Whitehall papers, according to some MPs who viewed them, bolstering the case for the Best for Britain projections.

The document also set out which regions will be hit the hardest, with the Leave-voting North-east of England and West Midlands expected to be the worst affected.

In a further blow to the Government, it also warned that the gains from free trade deals with other big countries would fail to compensate – a boost of just 0.2 per cent over 15 years from a deal with the US, for example.

A Government spokeswoman said the document was a "provisional internal analysis, part of a broad ongoing programme of analysis, and further work is in progress".