THERESA May has slapped down Boris Johnson over his challenge to her Chequers Plan as Downing Street brushed aside a poll, which suggested Brexit posed an existential threat to the Union.

The former Foreign Secretary, in what appeared to be the opening salvo in a bid to oust his party leader from No 10, claimed her compromise plan meant Britain had "gone into battle with the white flag fluttering over our leading tank". He said the Chequers Plan was a "fix" that could only lead to a victory for Brussels.

But when asked about Mr Johnson’s remarks, written in a Daily Telegraph article, the Prime Minister’s spokesman said: “Boris Johnson resigned over Chequers. There are no new ideas in this article to respond to. 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” proposal, which had been put forward and which would deliver on the will of the British people.

The spokesman claimed progress had been made over the summer with several European leaders speaking positively about the compromise proposals.

“This is a negotiation. What the PM has said very clearly is we have made our move and it’s time for the EU to make its move…We want the EU to engage in these negotiations with energy and imagination.”

But when it was suggested Michel Barnier, the EU’s chief negotiator, had rubbished the Chequers Plan, branding the UK Government’s customs proposals at the weekend “illegal,” the spokesman said: “Michael Barnier has said a lot of things in recent weeks.

“The negotiations between the Secretary of State and Michel Barnier are ongoing and you can expect some to be taking place in the current days. It’s very clear the Chequers proposals are being taken seriously; that’s why those talks are taking place.”

When asked if David Lidington, Mrs May’s effective deputy, was right to say last week that it was the Chequers Plan or no deal, the spokesman said: “The PM has set out the proposal, which can deliver on the will of the British people and meet those important objectives of taking back control of our borders, money and laws while also ensuring there is no hard border in Ireland.

“We have also said we are making preparations for a no-deal and we will have in place the structures we need that allow us to thrive in the future…The proposal we have put forward is negotiable and can carry the support of the House of Commons.”

At the weekend, a poll commissioned by the pro-Remain group Best for Britain, said 47 per cent of Scots would back independence if Brexit happened with 43 per cent opposed and 10 per cent saying they did not know. In Northern Ireland, 52 per cent of voters said they would back a united Ireland if Brexit happened and 39 per cent would vote to stay in the UK.

Asked if Mrs May feared Brexit posed an existential threat to the Union, her spokesman said: “The PM has been absolutely clear throughout about her determination to deliver Brexit in such a way that protects the integrity of the Union.

“The PM is absolutely clear that the proposals we bring forward shouldn’t lead to Northern Ireland being treated differently to any other part of the UK. It’s vital we maintain the integrity of the whole of the UK. She has been very clear in stressing the huge importance of that,” he added.

Earlier, Jeremy Corbyn was asked, on the back of Mr Johnson’s broadside, if the PM’s Brexit plan was in tatters. “It sounds like it is and it sounds like Boris Johnson, having spent two years as Foreign Secretary, has achieved nothing and now says it's all off.

"Well surely, it's time for some serious people to take over the negotiations? The Tories have had two years since the referendum and made no progress at all," said the Labour leader.

David Davis, the former Brexit Secretary, who like Mr Johnson resigned over the Chequers Plan, made a veiled attack on his former Cabinet colleague, telling Good Morning Britain now was not the time for "personality politics".

Asked if it would be better if Mrs May stood down, Mr Davis replied: "No, we don't need any more turbulence right now. What matters in all of this is not the personality politics, it's the outcome at the end."

Damian Green, the PM’s former de facto deputy, claimed Mrs May's position was "difficult but not impossible" and her Chequers Plan would end up winning support.

"We're walking a narrow path with people chucking rocks at us from both sides. Everyone is going to have to face the fact that the British Government has got a plan...No-one else in the EU has suggested a plan that is in any way workable," he said.

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

But he also made clear there should not be a challenge to her leadership, stressing now was “not the time to start monkeying around changing leader”.

No 10 dismissed Mr Boles’s proposal, saying it would not deliver on the wishes of British voters expressed in the 2016 referendum.

“The British public voted to take back control of its borders and bring an end to free movement. Membership of the EEA doesn't allow for that," it said.