Michel Barnier savaged Theresa May's Chequers plan for Brexit today, warning it would undermine the EU single market.

The EU negotiator's warning in the German press came just hours after ex-Brexit secretary David Davis vowed to vote against the plan on the grounds it would be worse than Britain staying in the bloc outright.

The two interventions from Brussels and Brexiteers underline the challenge Mrs May faces in getting any kind of deal based on the proposals.

Labour is also thought likely to vote against the Brexit deal secured by the Prime Minister - raising the prospect Mrs May could fail to get an agreement at all.

EU negotiator Michel Barnier renewed his criticism of Mrs May's proposals today, warning they would undermine the foundations of the block - underlining how unpopular the Chequers plan has been on all fronts.

In his new intervention, Mr Barnier told Germany's FAZ newspaper the Chequers plans would undermine the EU.

He said: 'The British have a choice. They could stay in the single market, like Norway, which is also not a member of the EU, but they would then have to accept all the regulations and make contributions to European solidarity.

'But if we let the British cherrypick which of our regulations to follow, that would have serious consequence: all sorts of other third countries could insist that we offer them the same deal.

'That would be the end of the single market and the European project! I am often accused of being dogmatic in the UK, but the truth is I'm only protecting our fundamental interests.'

Former Brexit Secretary David Davis (pictured today at the BBC) warned the Prime Minister today he vote against deal based on her Chequers plan

Interviewed by the BBC's Andrew Marr today Mr Davis said the blueprint hammered out at Chequers - which forced him to resign from the Cabinet - was 'worse than remaining in'.

And he said getting a deal based on the proposal would require even more concessions from Britain - claiming her promise to only make adjustments to the plan in the 'national interest' was like saying 'open sesame' to Brussels.

He said: 'No I'd vote against it, it would be rather odd for me to resign over something and then vote for it when it came back.

'In my view the Chequers proposal - it's not a deal, we shouldn't call it the Chequers deal, it's the Chequers proposal, is actually almost worse than being in.'

Asked if Theresa May could stay on as Prime Minister if she could not get the Chequers plan through the Commons, he said: 'Oh yes.'

Outlining why Chequers was worse than membership, Mr Davis added: 'We will be under the rule of the EU with respect to all of our manufactured goods and agri-foods, that's a really serious concession, what about take back control, it doesn't work.

Fox jibes at Hammond long term Treasury forecasts are always wrong International Trade Secretary Liam Fox dismissed the Chancellor's pessimistic forecasts of a no deal Brexit today. He was challenged about Philip Hammond's comments on the possible economic impact of going on WTO rules in the event of a no deal. Mr Hammond has been slammed by Brexiteers for promoting pessimistic forcecasts about the economy after no deal. Dr Fox told Marr: 'Can you think back in all your time in politics where the Treasury have made predictions that were correct 15 years out, I can't, they didn't predict the financial crisis that happened, no-one could. 'And so this idea that we can predict what our borrowing would be 15 years in advance is just a bit hard to swallow.' Advertisement

'That actually leaves us in a position where they dictate our future rules without us having a say at all, so it's a worse deal.'

Mr Davis added: 'The other argument I made at that Chequers cabinet was this won't be the last step, they will not accept this.'

Reacting to Mrs May's statement that there won't be more compromises, he said: 'Except in the national interest... that is an incredible open sesame to all.'

On the Northern Ireland border, he added: 'I do think, I'm free to talk more freely now than perhaps when I was a minister, I do think we've heavily over-emphasised the problem on the Northern Ireland border...

'This is a much more straightforward issue to deal with if we choose to, if we put the political will behind it, we and the Irish Republic, the two together.'

Mr Davis argued Brexit was about facilitating trade with the rest of the world outside the EU.

He said: 'I said all along there'd be technical and tactical problems and there'd be pressure points and we'll have to live through the pressure points, we have to see the other side off in some respects here.'

Mr Davis told the BBC's Andrew Marr (pictured) the blueprint hammered out at Chequers - which forced him to resign from the Cabinet - was 'worse than remaining in'

Mr Davis went on: 'I was always the Brexit Secretary, the question is whether I controlled events, that's another matter.'

He added: 'The reason I left after Chequers was we'd got to a point where her (Mrs May's) view advised by Olly (Robbins), but not just by Olly, advised by many others was different from mine.'

Asked if the Prime Minister believed Brexit would be a good thing for Britain he said: 'I think so.'

In other developments today, International Trade Secretary Liam Fox was challenged about Chancellor Philip Hammond's comments on the possible economic impact of going on WTO rules in the event of a no deal.

Mr Hammond has been slammed by Brexiteers for promoting pessimistic forcecasts about the economy after no deal.

International Trade Secretary Liam Fox (pictured today on Marr) was challenged about Chancellor Philip Hammond's comments on the possible economic impact of going on WTO rules in the event of a no deal.

Dr Fox told Marr: 'Can you think back in all your time in politics where the Treasury have made predictions that were correct 15 years out, I can't, they didn't predict the financial crisis that happened, no-one could'

Dr Fox told Marr: 'Can you think back in all your time in politics where the Treasury have made predictions that were correct 15 years out, I can't, they didn't predict the financial crisis that happened, no-one could.

'And so this idea that we can predict what our borrowing would be 15 years in advance is just a bit hard to swallow.'

Dr Fox also played down threats to Mrs May's position, warning Brexiteer rebels that changing the Prime Minister would not change the 'parliamentary arithmetic' - meaning they would have no greater chance of getting a hard Brexit through Parliament without gambling on a new general election.

Earlier, Mrs May today ruled out giving in to calls for a second EU referendum warning it would be a 'gross betrayal of our democracy'.

The Prime Minister said voters made their decision in 2016 by 52 to 48 per cent and that must be the end of the debate of whether Britain will leave the European Union.

Pro EU campaigners have spent the summer building momentum and resources behind a call for a new referendum once the Brexit deal has been negotiated.

The People's Vote campaign insists they want a 'first referendum on the facts' but it has enraged Brexiteers who say the issue has been settled.

Theresa May (pictured on Thursday in Kenya) today ruled out giving in to calls for a second EU referendum warning it would be a 'gross betrayal of our democracy'

Mrs May moved to close down the question today in comments revealed by the Sunday Mirror.

She said: 'I will not give in to those who want to reopen the whole question with a second referendum.'

'They trusted that their vote would count, that after years of feeling ignored by politics, their voices would be heard.

'To ask the question all over again would be a gross betrayal of our democracy - and a betrayal of that trust.'

People's Vote supporter Chuka Umunn said: ''The real betrayal of trust would be to deny the people a say on any Brexit deal which is nothing like the one they were promised.

'It is a betrayal of democracy for Theresa May to force a bad deal - or no deal - on Britain without giving the public the chance to have a final say in a People's Vote.

'More and more people from all walks of life and every corner of the country are demanding their democratic voice is heard. It is time for Westminster's leaders to listen.'

The People's Vote campaign (which has cross party support from Lib Dem Layla Moran, Labour's Chuka Umunna and Green Caroline Lucas, pictured) insists they want a 'first referendum on the facts' but it has enraged Brexiteers who say the issue has been settled

The Prime Minister has stood by the Government's controversial Chequers exit plan despite continued sniping against it from Tory benches.

Writing in the Sunday Telegraph, Mrs May said: 'I will not be pushed into accepting compromises on the Chequers proposals that are not in our national interest.'

The comments came as it was reported that the election strategist who helped Boris Johnson win the London mayoralty was involved in a bid to scupper the Cabinet's Chequers compromise.

Sir Lynton Crosby, who also advised Mrs May in last year's general election when the Tories lost their majority in a shock poll result, is manoeuvring to derail the PM's EU withdrawal agenda with a co-ordinated national campaign, according to the Sunday Times.

The claims emerged as former minister and high-profile Tory MP Nick Boles, who backed Remain at the referendum, came out against the Chequers deal.

He told the Sunday Telegraph that under current Government plans, the UK faces 'the humiliation of a deal dictated by Brussels', which is treating the Chequers proposals as an 'opening bid'.