THERESA May faces a constitutional crisis unless she guts the legislation intended to deliver Brexit, Scottish ministers have bluntly warned her deputy.

In a bracing introductory meeting in Edinburgh, First Secretary Damian Green was told Holyrood would reject the EU (Withdrawal) Bill out of hand unless “serious and significant changes” were made to a key section on where powers would lie after Brexit.

The SNP Government claims the Bill, as drafted, would result in powers in 110 devolved policy areas repatriated from Brussels to London instead of returning to Holyrood.

The meeting was effectively the start of talks to find a figure acceptable to both sides.

An SNP source suggested cutting the number of devolved policy areas going to Westminster to 15 or 20 could change minds at Holyrood.

Although Holyrood cannot to block the Bill or Brexit, if MSPs withheld legislative consent it would be unprecedented for Westminster to ignore them and press on regardless.

A constitutional crisis over the devolution settlement would almost certainly follow.

SNP Brexit minister Michael Russell and deputy First Minister John Swinney met Mr Green and Scottish Secretary David Mundell at St Andrew's House in Edinburgh.

The UK Government described the encounter as “cordial” and said positive steps had been made towards working out the redistribution of powers once the UK leaves the EU.

The Brexit Bill says all powers currently exercised at EU level will initially be repatriated to Westminster, even those in devolved areas such as farming and agriculture.

Some powers would then be “released” to Holyrood, but others would be subject to UK-wide “common frameworks” to safeguard the UK’s internal market.

The aim of the meeting was to start identifying which powers could go to the Scottish Parliament relatively quickly, and which dealt with at a UK level.

Mr Russell said the meeting was “a useful opportunity for an exchange of views”.

But he added: “We remain absolutely clear that, as things stand, we will not recommend to the Scottish Parliament that it gives its consent to the EU Withdrawal Bill.

“The bill as currently drafted is impractical and unworkable.

“It is a blatant power grab which would take existing competence over a wide range of devolved policy areas, including aspects of things like agriculture and fishing, away from Holyrood, giving them instead to Westminster and Whitehall.

“Unless there are serious and significant changes to the proposed legislation, the strong likelihood is that the Scottish Parliament will vote against the repeal bill.

“We have never claimed to have a veto over EU withdrawal.

“But UK Ministers should be in no doubt - to override a vote of the Scottish Parliament and impose the Withdrawal Bill on Scotland would be an extraordinary and unprecedented step.

“The current proposals are a direct threat to the devolution settlement which the people of Scotland overwhelmingly voted for in 1997.

“We are not opposed in principle to UK-wide frameworks in certain areas – but this must be on the basis of agreement among equals, not imposed by Westminster.”

Mr Green said: “I thought the talks were positive. There are issues on which the UK and Scottish Governments place a different emphasis, but we agreed that we need to work on the principles on which we’ll engage.

“We were also united on the aim that the Scottish Parliament has more powers at the end of this process, and on the importance of preserving free trade within the United Kingdom.

"We’ve agreed more talks in a few weeks time, and I am confident that we’ll be able to reassure the Scottish Government further of our good faith on these matters."

Meanwhile, a former chief of staff to Brexit Secretary David Davis has said a new party may be the best way to fight the “catastrophe” of Brexit.

James Chapman, who also worked for former Chancellor George Osborne, said it was “past time for sensible MPs in all parties to admit Brexit is a catastrophe, come together in a new party if necessary, and reverse it”.

Floating the name The Democrats for the party, he claimed some “very interesting people” were sympathetic to the idea.

Mr Chapman also used Twitter to push the UK government for answers on problems with air travel, cancer treatment, and border checks as a result of the UK’s leaving the EU.