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The UK faces a "full-blown constitutional crisis" unless all four nations agree a plan for Brexit, a leading think tank has warned.

The stark warning comes as Theresa May holds her first summit today with the leaders of Scotland, Wales and Northern Ireland.

Nicola Sturgeon, Carwyn Jones and Arlene Foster will meet the PM and Brexit Secretary David Davis in Downing Street in the first meeting of its kind for two years.

Downing Street dismissed suggestions the devolved assemblies could have a final say on the negotiation package.

But the Institute for Government (IfG) warned they should agree "core planks" of the negotiation before the two-year exit process.

(Image: Getty)

It warned forcing a deal on the nations may be legally possible but would be a "reckless strategy" which could lead to the possible break-up of the UK.

"There is little common ground between these leaders on the future of the UK or almost anything else," the IfG's report said.

"So, as with the dog walking on its hind legs, perhaps we should be impressed if the four governments work together at all rather than overly critical of how skilfully they perform the task.

"But the stakes are high. If it proves impossible to find consensus and the dog topples over after a few tentative steps, the result may be a serious breakdown in relations between the four governments (and nations) of the UK."

In a growing row, Scottish First minister Ms Sturgeon and Welsh leader Mr Jones will tell the Prime Minister their national Parliaments should have a say on the proposed deal.

The meeting comes as the British Bankers’ Association warned several banks were poised to quit the UK because of Brexit.

Mrs May will meet with Ms Sturgeon, Mr Jones and Northern Ireland’s First Minister Arlene Foster in Downing Street to discuss their concerns about leaving the EU.

(Image: PA)

In a warning shot, Ms Sturgeon said all three devolved administrations were in favour of the UK remaining in the single market.

“While there are some areas, such as continued UK participation in the single market, on which all three devolved administrations will agree, the UK position will also need to take account of the specific needs, responsibilities and priorities of each administration as part of the overall UK package.

“To ensure those are properly reflected and the stated position of all parts of the UK as a partnership of equals is respected, I also endorse the proposal that the proposed negotiating package should be subject to a vote in each of the four of the United Kingdom’s parliaments and assemblies,” Ms Sturgeon said in a letter to the PM.

Mrs May is to set up a new committee, chaired by Brexit Secretary David Davis, that will meet at least twice before she triggers Article 50 - the mechanism for leaving the EU - in the New Year.

(Image: Western Mail)

The Prime Minister said: “The country is facing a negotiation of tremendous importance and it is imperative that the devolved administrations play 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.”

The Government was also warned by the head of the British Bankers’ Association, Anthony Browne, that several UK-based banks were on the verge of quitting the country because of Brexit.

“Their hands are quivering over the relocate button. Many smaller banks plan to start relocating before Christmas, bigger banks are expected to start in the first quarter of next year,” Mr Browne said.

It was also reported that the Government is looking at cutting corporation tax from 20% to 10% to encourage firms to stay in the UK.

Tory MP and pro-Remainer Dominic Grieve MP said the warning from the Bankers’ Association was “worrying.”

“Jobs across the country are dependent on our having thriving financial services. They should be supported with a clear statement from Ministers that we will remain in the EU’s single market. Poor substitute arrangements could be damaging.

“This uncertainty and speculation underlines why the Government needs to provide clarity on its direction of travel, with greater willingness to discuss painful trade-offs to come.”