A General Election should be called if Holyrood and Cardiff reject Theresa May’s Brexit deal, Carwyn Jones, the First Minister of Wales, has insisted.

And the Labour leader made clear that a rejection of a Brexit deal by either the Scottish Parliament or Welsh Assembly could also eventually trigger a second referendum on the UK's relationship with the EU.

Wales voted to leave the bloc by a margin of 52.5 to 47.5 per cent in the 2016 referendum, although recent polls have suggested a swing towards remain. In Scotland, there was a strong majority – 62 to 38 per cent – to remain.

Setting out what he believes should happen if the Prime Minister secured a withdrawal agreement with Brussels, Mr Jones said: "The next step is to see whether a deal can be supported by, for me, parliaments plural: Westminster, Edinburgh and Cardiff.

"If that doesn't happen, then I don't see any alternative other than a general election where Brexit would be the main issue.

"If there's an inconclusive result as a result of that general election, well, how then do you resolve the issue without going back to the very same people who took the decision in the first place? At that point you are talking about the potential for a second referendum," added the AM for Bridgend.

His remarks came as battle-lines were being drawn ahead of this evening’s informal EU summit in Salzburg when Mrs May will seek to convince her fellow leaders of her Chequers Plan.

But ahead of it, Whitehall sources suggested that she had already rejected the “improved” offer on the Irish border mooted by Michel Barnier, the EU’s chief negotiator.

One insider explained that Mr Barnier had still not dropped his view that Northern Ireland should be treated as a separate customs jurisdiction from the rest of the UK.

“Any backstop will have to respect our red line that there is not a customs border in the Irish Sea. Mr Barnier has not said anything to suggest that his position has changed on that,” said the source.

Nigel Dodds of the Democratic Unionist Party, which props up Mrs May's minority administration in Parliament, gave Mr Barnier's proposals short shrift.

"It still means a border down the Irish Sea although with different kinds of checks," said the party's deputy leader, adding: “The fact is that both Theresa May and the Labour Party have said no British prime minister could accept such a concept. It is not just Unionists who object."

Meanwhile, Donald Tusk made clear the UK’s Brexit plans needed to be "reworked".

The European Council President told reporters in Salzburg: "The Brexit negotiations are entering the decisive phase. Various scenarios are still possible today but I would like to stress that some of Prime Minister May's proposals from Chequers indicate a positive evolution in the UK's approach, as well as the will to minimise the negative effects of Brexit."

Those positive areas included foreign and security policy co-operation, he explained.

But in a blow to Mrs May's approach, Mr Tusk noted: "On other issues such as the Irish question or the framework for economic co-operation the UK's proposals will need to be reworked and further negotiated.”

He added that perhaps there was now “more hope but there is surely less and less time".

According to extracts released ahead of Mr Davis's speech, the former Cabinet minister will say: "We have been told that the Chequers proposal fulfils what the British people voted for. Well, I am afraid I simply do not buy that.

"Fifty-two per cent of British voters oppose the proposals. Only 18 per cent approve. It is quite remarkable for a government policy to be that unpopular. And, for me, the Chequers plan was always a non-starter.”

He noted how in her speeches at Lancaster House and at the Mansion House, the PM had promised to return control over the country’s laws, money and borders. These promises, he stressed, were in the Tory manifesto too, yet the Chequers Plan crossed all of those red lines.

"The EU is often correctly described as having a democratic deficit. But Chequers is devoid of democracy altogether.

"This is why many of us will shortly be presenting an alternative plan which will outline a more ambitious vision."

Mr Davis will go on to say that German companies should not fear a more competitive UK outside the EU but, rather, should "embrace the opportunity that it will create for German workers, families and businesses large and small".

"In the long run, the friendly economic competition between neighbours is massively beneficial.

"Beneficial for the German companies whose supply chains will be provided with improved components from British factories, beneficial for the consumers of tomorrow, who will enjoy the fruits of a more competitive global economy, and beneficial for the students and entrepreneurs of all Europe," the Yorkshire MP will add.