THERESA May's Chequers Plan is “more unpopular than the poll tax," a former Cabinet minister has insisted, as the Tory war on Brexit intensified.

Conservative Remainers and Leavers now agree on one thing: the Prime Minister's Brexit compromise is dead.

As MPs return to Westminster today following their summer recess, Dominic Raab, the Brexit Secretary, is expected to deliver an update on the talks with Brussels. With the October deadline looking impractical, a special European summit in November now seems likely.

Yesterday, the Conservative in-fighting got personal after Boris Johnson – in what appeared to be the opening salvo in his bid to oust his party leader from No 10 - claimed the PM’s compromise proposals meant Britain had "gone into battle with the white flag fluttering over our leading tank". The former Foreign Secretary said the Chequers Plan was a "fix" that can only lead to victory for Brussels.

But No 10 hit back. The PM’s spokesman, noting how Mr Johnson had offered “no new ideas,” slapped down the former Cabinet minister, saying: “What we need at this time is serious leadership with a serious plan and that's exactly what the country has with this Prime Minister and this Brexit plan…She is a serious prime minister and she is putting forward serious proposals.”

He argued that Mrs May’s Chequers Plan was the “only credible and negotiable” Brexit blueprint, which had been put forward and which would deliver on the will of the British people.

However at the weekend, Michel Barnier launched an excoriating attack on the PM’s proposals, saying he was strongly opposed to them and, if implemented, they would be the “end of the single market and the European project".

Emerging from a meeting with the EU’s chief negotiator as part of the Commons Brexit Committee, Jacob Rees-Mogg, the arch-Brexiteer, said: “We found ourselves in considerable agreement that Chequers is absolute rubbish and we should chuck it and have a Canada-style trade agreement instead.

“Eurosceptics and Monsieur Barnier are in greater agreement than Eurosceptics and the Government or Monsieur Barnier and the Government. It is very encouraging.”

The Somerset MP also defended Mr Johnson, noting: “Downing Street only criticises politicians about whom it is affeared.”

Earlier, Tory Remainers also came out against the Chequers Plan.

Nicholas Boles, the former UK Housing Minister, who has suggested replacing the 20-month transition with a limited period in the European Economic Area before moving to a free trade deal, argued there was "as close to zero" chance of the PM winning support from Parliament for her compromise proposal.

His Conservative colleague, Justine Greening, the former Education Secretary, declared: “The Chequers deal is now more unpopular than the poll tax. It is now dead and there's no point having a government spending two vital months working on that when what it needs to do is start working on an alternative route for us."

One Scottish Remainer Tory MP decried Mr Johnson’s intervention as “completely irrelevant” but agreed that the Chequers Plan was a non-starter given the EU’s deep opposition to the proposed customs arrangements.

He said it was looking likely that there would be a no-deal but that it would be a “negotiated no-deal,” covering various policy areas and which would still take two years to implement. “I would be happy with a Norway-style option,” he said.

A Scottish Brexiteer colleague also thought the Chequers Plan was dead as Mr Barnier had come out so strongly against the proposed customs arrangements. “The EU are never going to accept them. It’s going to end up as a free trade agreement; Canada plus plus plus,” he added.

Meanwhile, the Scottish Government has highlighted "serious concerns" over the possibility of a no-deal Brexit in a letter to Mrs May's effective deputy.

Michael Russell, Scotland's Constitutional Relations Secretary, claimed such an outcome would be "irresponsible" and would greatly increase the uncertainties over Britain's exit from the EU.

In his letter to David Lidington, he warned of "significant negative economic and social consequences" following any no-deal result and stressed that the devolved administrations' views must inform UK preparations for such an event.

On September 13, it is expected the UK Cabinet will hold a special meeting to look at "no deal" preparations. Mr Russell said it was "vitally important that that type of discussion is informed by the views of the devolved administrations".