Labour has opened up its biggest lead over the Tories since the general election, according to polls carried out after the prime minister unveiled her Brexit plans.

The fallout from the controversial Chequers summit appears to have caused a drop in support for the Conservatives and a revival of fortunes for UKIP.

A poll carried out for the Observer by Opinium on 10 July puts the Tories on 36 per cent, a fall of six points since June, while Labour remained on 40 per cent. UKIP rose by five points to eight per cent.

If the results were repeated at a general election, the Tories would lose around 50 seats and Labour would be the largest party, albeit 26 seats short of a majority.

Another poll, by Deltapoll for The Sun on Sunday, puts the Tories on 37 per cent and Labour on 42 per cent.

It also suggested that 62 per cent of voters think Theresa May should stand down before the next general election.

Only seven per cent of voters believed the Chequers plans was the best Brexit solution.

Ian Lavery MP, chair of the Labour Party, said: “Theresa May’s so-called plan doesn’t stand up to scrutiny.

“The Tories are botching Brexit, so the Prime Minister has resorted to shameless threats aimed at the British people.

“No one – not the public, Parliament or the Conservative party – is happy with Theresa May’s offer. This has descended into a shambles.”

Steve Baker MP, who resigned as Brexit minister after the Chequers summit, greeted the polls by tweeting: "It looks like Chequers means Corbyn."

Tory MP Jacob Rees-Mogg told The Sun: "This poll demonstrates, unsurprisingly, that voters have seen through the surrender paper."

Theresa May has warned rebel Conservative MPs that Brexit might not happen at all if they do not back her plans.

“We need to keep our eyes on the prize," she wrote in the Mail on Sunday. "If we don’t, we risk ending up with no Brexit at all."

Trips to Chequers: a history of famous visits Show all 14 1 /14 Trips to Chequers: a history of famous visits Trips to Chequers: a history of famous visits David Cameron hosts Chinese President Xi Jinping in 2015. A notable visit if only for the pictures of David Cameron looking awkward drinking a pint AFP/Getty Trips to Chequers: a history of famous visits Edward Heath and the Queen host then US President Richard Nixon and his wife at Chequers in 1970 PA Trips to Chequers: a history of famous visits Former Soviet heavyweight Mikhail Gorbachev visits Margaret Thatcher at Chequers in 1984 Getty Trips to Chequers: a history of famous visits David Cameron hosts German Chancellor Angela Merkel in 2015. Mrs Merkel had previously visited during Gordon Brown's time as PM AFP/Getty Trips to Chequers: a history of famous visits Gerry Adams and Martin McGuiness of Sinn Féin visit Chequers in 2003 PA Trips to Chequers: a history of famous visits President of the European Commission Jean-Claude Juncker visits Chequers in 2015 to renegotiate the UK's EU membership Getty Trips to Chequers: a history of famous visits Prince Philip visits in 2014 and plants a tree with David Cameron Getty Trips to Chequers: a history of famous visits Then US Secretary of State Hillary Clinton is hosted by Gordon Brown in 2009. Mrs Clinton had previously visited in the 1990s as First Lady of the United States Getty Trips to Chequers: a history of famous visits Russian President Vladimir Putin is received at Chequers by Tony and Cherie Blair AFP/Getty Trips to Chequers: a history of famous visits Margaret Thatcher hosts Zimbabwean President Robert Mugabe in 1988 PA Trips to Chequers: a history of famous visits George Bush Sr. visits Thatcher at Chequers in 1984 PA Trips to Chequers: a history of famous visits President Dwight D. Eisenhower visits Harold Macmillan at Chequers in 1959 PA Trips to Chequers: a history of famous visits Ramsay Macdonald hosts W.T Cosgrave, President of the Executive Council of the Irish Free State and Sir James Craig, Prime Minister of Northern Ireland , at Chequers in 1924 to negotiate the Irish border PA Trips to Chequers: a history of famous visits David Lloyd George, the first Prime Minister to use Chequers, hosts French Prime Minister Aristide Briand and Marshall Foch in 1921 Getty

Her warning comes after both Boris Johnson and David Davis resigned in protest at the Chequers plan and Donald Trump criticised her approach to Brexit during his official visit to the UK.

Ms May has revealed that the US president advised her to sue the EU rather than go into negotiations.