An alliance of Remainer MPs are believed to be secretly plotting to revoke Article 50 to stop the UK leaving the EU on October 31.

If a deal can't be agreed between Boris Johnson and EU leaders by the Brexit date, the government has warned some MPs will try to force through new legislation to prevent departure from the Union.

It is believed the alliance involves MPs from different parties and Tony Blair and Peter Mandelson are 'pulling the strings' behind the scenes.

It came as the Liberal Democrats became a party of out-and-out Remain today, with its members voting to put a stop to Brexit without a new referendum in the unlikely event it wins the next election.

Jo Swinson's party was previously campaigning for a second Brexit referendum, but it has now pivoted to out-and-out support for ignoring the 2016 referendum.

A motion overwhelmingly passed at the event in Bournemouth today says a Liberal Democrat majority government would be 'recognised as an unequivocal mandate to revoke Article 50 and for the UK to stay in the EU'.

An alliance of Remainer MPs are attempting to thwart Boris Johnson's Brexit plans by secretly plotting to revoke Article 50

Former prime minister Tony Blair and his sidekick Peter Mandelson, dubbed the 'Prince of Darkness' are believed to be pulling the strings behind the Remainer alliance

Delegates at the Lib Dem conference in Bournemouth this afternoon voted overwhelmingly to support a motion to revoke Article 50 if it takes power

Boris Johnson is preparing to fly to Luxembourg to tell Jean-Claude Juncker he is 'striving' to agree a deal before next month's EU Council meeting on October 17 and 18.

A source told The Telegraph: 'The PM will not negotiate a delay at the Brussels council.

'We expect there to be a major court battle immediately after the 19th and attempts to pass legislation revoking Article 50, which the Prime Minister will refuse to consider in any circumstances.'

Ms Swinson told the BBC today: 'If the Liberal Democrats win a majority at the next election, if people put into government, as a majority government, the ''Stop Brexit'' party, then stopping Brexit is exactly what people will get'

Former prime minister Blair and his sidekick, dubbed by opponents as the 'Prince of Darkness', are alleged to be giving secret advice to the cross-party alliance.

I cannot forgive David Cameron for calling the Brexit referendum, says Jo Swinson Liberal Democrats leader Jo Swinson said she cannot forgive former prime minister David Cameron for calling the 2016 Brexit referendum. Mr Cameron apologised this week for failing to unite the country during that vote when he was Tory leader. Speaking at the Lib Dem Bournemouth conference, Ms Swinson said: 'No, I don't forgive David Cameron for calling the referendum. "I think so much of the problems we are facing right now stem from David Cameron's shocking misjudgment in putting the interests of the Conservative Party in front of the national interest.' Advertisement

A source said: 'Blair and Mandelson are pulling the strings. Mandelson is the puppet master, he has had meetings via proxy with all the parties involved in the Remain alliance and sends emails with ideas.

'Blair's people have been speaking to Jo Swinson's people.'

Ms Swinson told the BBC's Andrew Marr Show today: 'The policy we are debating at conference today is very clear.

'If the Liberal Democrats win a majority at the next election, if people put into government, as a majority government, the ''Stop Brexit'' party, then stopping Brexit is exactly what people will get. Yes, we will revoke Article 50.

The pro-EU party is enjoying a revival under Ms Swinson's watch, having seen a host of MPs defect to her party with its clear stance on Britain's relationship with Brussels.

Sam Gyimah, a former Tory minister, became the sixth MP to switch allegiance to the party this year and some polling companies predict the Lib Dems could take as much as a fifth of the vote at the next election - up from just 7% in 2017.

And in a bid to ensure the Lib Dems go into the next election as the most fervently Remain-supporting party, Ms Swinson is asking members to vote on whether to adopt revoking Article 50 as its central Brexit policy at its conference in Bournemouth on Sunday.

Last night the former universities minister Sam Gyimah defected to the Liberal Democrats in another blow to the new prime minister

East Dunbartonshire MP Ms Swinson added: 'We have argued that a specific Brexit deal should be put to a People's Vote to give clarity.

'We still argue for that. But if we end up at a general election then I think we need to be straight forward with people and give them an option for all this Brexit chaos to stop.

'I recognise not everyone agrees with the Lib Dems on this. (But) it is genuinely what we think is right for the country.'

James Cleverly (left) blasted the Lib Dems for trying to ignore the Brexit vote, hours after former Tory leadership contender Sam Gyimah (right) defected to Jo Swinson's party

Boris Johnson suffered another blow last night as another Tory rebel defected to the Liberal Democrats.

Sam Gyimah, the former universities minister, had had the whip removed after voting against the Government and joined Jo Swinson, the Lib Dem leader, on stage at their party conference in Bournemouth.

The new prime minister will also face a three-day legal challenge this week over his decision to prorogue Parliament.

The decision, which will be taken by the Supreme Court, could see MPs returning to work in the first week of October if the government loses.

David Cameron blasts 'lying' Leave leaders: Ex-PM says Boris Johnson 'didn't believe' in Brexit and suggested a second referendum, while Michael Gove is a 'foam-flecked Faragist' who is an 'ambassador for truth-twisting populism

David Cameron blasted his Tory successor Boris Johnson today, accusing the Prime Minister of being a fake Brexiteer who only backed Leave to further his political ambition.

The former leader, who quit after leading the failed Remain campaign in 2016, also lashed out at Michael Gove, who co-led Vote Leave, labelling his former close friend a 'foam-flecked Faragist'.

In extracts from his autobiography, serialised in the Sunday Times, Mr Cameron tore into Mr Johnson saying he 'didn't believe' in Brexit and had privately claimed there could be a 'fresh renegotiation, followed by a second referendum' - which he now says he opposes.

And Mr Cameron accused the leaders of the Leave campaign of declaring 'open warfare' on him - and claimed they were guilty of 'lying' to the public to win the 2016 referendum.

As well as the ministers he also tore into Dominic Cummings, the brains behind the Leave campaign and now a senior adviser to Mr Johnson in No 10, along with Brexit Party leader Nigel Farage, then of Ukip.

He described them as part of a 'cauldron of toxicity' who had 'something of the night about them'.

Mr Cameron's bitter criticism comes as Mr Johnson warned Brussels that Britain will break out of its 'manacles' like The Incredible Hulk if a Brexit deal cannot be struck by October 31.

In an exclusive interview with The Mail on Sunday, the Prime Minister said that if negotiations break down, he will ignore the Commons vote ordering him to delay the UK's departure, adding: 'The madder Hulk gets, the stronger Hulk gets.'

His bullish declaration comes ahead of a crunch meeting tomorrow with European Commission chief Jean-Claude Juncker.

David Cameron has launched another stinging attack on Boris Johnson, claiming the Prime Minister 'didn't believe' in Brexit and only backed the Leave campaign to boost his career

In fresh extracts from his controversial book, and in a stunning blue-on-blue rant, Mr Cameron claimed:

Boris Johnson privately said there could be a second referendum following fresh Brussels negotiations;

The current PM had never 'believed' in Brexit but joined Vote Leave to help secure the keys to No10;

Michael Gove appeared to ape Nigel Farage by campaigning against immigration despite his liberal background, becoming a 'foam-flecked Faragist' during the referendum, despite promising Mr Cameron he would only pay a minor role in the battle;

That he felt personal failings for the result after failing to publicly promise to block Turkey from joining the EU - a high-profile campaign issue in 2016;

That Mr Johnson's current Downing Street chief Dominic Cummings had 'something of the night about him' and was a 'cauldron of toxicity'.

That he under-estimated the 'latent Leaver gene' in Tory MPs including Priti Patel and Liz Truss

Mr Cameron wrote that the now-Prime Minister wanted to become the 'darling of the party' and 'didn't want to risk allowing someone else with a high profile - Michael Gove in particular - to win that crown'.

Tennis and fireside Chequers chats: how David Cameron tried to win over Boris Johnson and Michael Gove David Cameron desperately tried to woo Boris Johnson into backing Remain after a private tennis date, offering him a senior Cabinet post and a chance to follow him into Downing Street, his memoir reveals. The former prime minister told how he offered to make the Eurosceptic defence secretary after a match at the US ambassador's private court, telling him that 'I'm not going to be prime minister forever' and 'you've got every opportunity to win' a leadership battle against George Osborne. Mr Cameron also told how he tried to woo Michael Gove via a fireside chat with his wife during a New Year's Eve party at his country home, Chequers - saying he was assured the then Justice Secretary would be on his side. But his attempts with both men were doomed to failure. A flurry of text exchanges with Mr Johnson followed their tennis match, Mr Cameron revealed, as Mr Johnson asked about getting clarity about British law trumping EU law. But Mr Cameron claimed it was clear the then backbench MP was mainly interested in 'what was the best outcome for him', adding; 'He risked an outcome he didn't believe in because it would help his political career'. He was even more shocked by the behaviour of 'close confidant' Mr Gove. According to the autobiography, at a meeting in Downing Street Mr Gove revealed 'my head is in a strange place', but pledged to limit his activity to 'one speech' if he did decide to back Brexit. Mr Gove went on to co-chair the Leave campaign. Advertisement

'The conclusion I am left with is that he risked an outcome he didn't believe in because it would help his political career.'

On Mr Gove, the former PM said: 'One quality shone through: disloyalty. Disloyalty to me and, later, disloyalty to Boris.'

And he said Mr Gove's claim that the public were tired of experts made him 'an ambassador for the truth-twisting age of populism'.

'By the end, Boris and Michael seemed to me to be different people. Boris had backed something he didn't believe in.

'Michael had backed something he did perhaps believe in, but in the process had broken with his friends ... while taking up positions that were completely against his political identity.

'Both then behaved appallingly, attacking their own government, turning a blind eye to their side's unpleasant actions and becoming ambassadors for the expert-trashing, truth-twisting age of populism.'

While Mr Cameron attacked his rival Mr Johnson as 'guilty of lying' during the 'open warfare' of the referendum campaign, book extracts published by The Sunday Times show the ex-Premier reserving most venom for Mr Gove.

Chronicling their Brexit fallout and his later stabbing of Mr Johnson in the back, Mr Cameron accused his former friend of having 'one quality' that 'shone through: disloyalty. 'Disloyalty to me and, later, disloyalty to Boris.'

And despite repeated denials from Mr Johnson regarding his support for a rerun of the Brexit vote, Mr Cameron stated that Johnson had privately said 'there could always be a fresh renegotiation, followed by a second referendum.'

Last night allies of the Prime Minister rejected the claim.