THERESA May has been warned that she risks causing a”full blown constitutional crisis” if she pursues a “reckless” strategy of trying to impose a Brexit settlement on Scotland.

The Prime Minister was given the stark warning by an independent think tank as Nicola Sturgeon travels to Downing Street for a crucial meeting about Britain’s withdrawal plans from Europe today.

But the Institute for Government warned Mrs May against imposing a Brexit deal on the devolved nations of Scotland, Wales and Northern Ireland without their consent as this would risk breaking up Britain.

Read more: Theresa May offers to involve Scotland in Brexit process

The group said in a new briefing paper, Four Nation Brexit, that although such a move was legally possible, it went against convention and “the spirit of devolution” that each nation should plot its own course.

It added: “This would run contrary to convention and to the spirit of devolution, which recognises the right of the three devolved nations to determine their own form of government.

“It would also be a reckless strategy for a government committed to the Union, since it would seriously undermine relationships between the four governments and increase the chances of Scottish independence and rifts in Northern Ireland’s fragile power-sharing arrangements.”

The Institute for Government argued the devolved governments should be treated as partners in the Brexit process not mere consultees alongside business or other lobby groups.

But it added: “However, this does not mean the four governments of the UK will have equal influence. The UK Government will lead the Brexit process and Westminster will retain the right to have the final say.”

The First Minister is due to take part in the first meeting of the Joint Ministerial Committee[JMC] alongside her counterparts from Wales and Northern Ireland this morning.

At the meeting, which the Prime Minister will chair, the First Minister and her colleagues will be offered a “direct line” to Whitehall as a sign of good faith to ensure the Scottish Government and the other devolved administrations have full engagement in the Brexit process.

No 10 said Mrs May is likely to those gathered that “contrary to some speculation, no final decisions have been taken and that how the UK leaves the EU will not boil down to a binary choice”.

Read more: Theresa May offers to involve Scotland in Brexit process

Ms Sturgeon, in a letter to Mrs May, said that Holyrood should have a vote on the UK’s negotiating position, which would be tantamount to a veto. The Conservative Government has already said this is a non-starter.

Over the weekend, the PM called for a “grown-up” relationship between London and Edinburgh and today will offer the FM and her Welsh and Northern Irish counterparts the chance of a “direct line to the Brexit secretary to allow them to help shape the UK’s EU exit strategy”.

This would involve the creation of a “new official forum,” a sub-committee of the JMC to be chaired by David Davis, the Brexit Secretary, with attendees nominated by the devolved governments. If agreed, the first meeting would take place by the end of November with at least one more before Christmas.

Mrs May said: “I am determined that as we make a success of our exit from the European Union, we in turn further strengthen our own enduring Union.

“The great Union between us has been the cornerstone of our prosperity in the past and it is absolutely vital to our success in the future.”

She stressed how the UK was facing a negotiation of tremendous importance and that it was imperative that the devolved administrations played their part in making it work.

“The new forum I am offering will be the chance for them all to put forward their proposals on how to seize the opportunities presented by Brexit and deliver the democratic decision expressed by the people of the UK,” added the PM.

In her letter, Ms Sturgeon, who declined a private meeting with Mr Davis when he visited Glasgow on Friday, noted how the four nations of the UK voted differently on June 23 with Scotland and Northern Ireland backing Remain while England and Wales supported Leave.

“We have a collective interest in seeing your commitment that the devolved administrations should be fully engaged in EU discussions and that you will not be triggering Article 50 until we have an agreed UK approach and objectives for negotiations, delivered in a full and meaningful fashion,” said the FM.

Read more: Theresa May offers to involve Scotland in Brexit process

Ms Sturgeon insisted that it would not be acceptable for the devolved administrations simply to be consulted on UK Government plans. “We must have meaningful input into the decision-making structure and the formation of negotiating positions.”

Calling for a clearly mapped out programme of involvement with the devolved administrations and a rigorous timeline, she added that to ensure the specific needs of Scotland, England, Wales and Northern Ireland were protected, the “proposed negotiating package should be subject to a vote in each of the four of the United Kingdom’s parliaments and assemblies”.

Meantime, former chancellor Lord Darling, in the wake of the publication by the Scottish Government of a Holyrood bill to hold a possible second independence referendum, said he did not believe Ms Sturgeon would hold another poll “any time soon” because she did not think she would win.

The onetime MP, who led the Better Together campaign in the 2014 plebiscite, said talking about the constitution was a “smokescreen” to distract from everything else that was going on.

Gordon Wilson, the former SNP leader, said he believed a second independence poll so soon could be a miscalculation and that Scotland needed to be more prepared before it voted again.

He said he thought Scotland needed to “get its expenditure trimmed down to the size a small country would want” and until it had done that, then another independence poll was “rather a waste of time”.