The think tank issued its warning after Scotland’s first minister Nicola Sturgeon this month unveiled plans to prepare for a second referendum on the country’s independence. It voted heavily to remain in the EU.
The IFG’s report Four Nation Brexit argued it would be imperative for Scotland, Northern Ireland, Wales and England reach agreement on the UK’s Brexit terms and negotiating strategy.
While May could impose a settlement on the devolved administrations this would be “a reckless strategy for a PM with a deep commitment to the Union”, it said.
“Scotland, Wales and Northern Ireland cannot be treated like any other lobby or interest group. Equally, the devolved governments will have to accept that Westminster will have the final say.”
It was “not reassuring” that May had so far said the negotiations were fore the UK government alone, the IFG added.
The report came as May, Sturgeon, Wales’ first minister Carwyn James and Northern Ireland’s first minister Arlene Foster and her deputy Martin McGuinness met to discuss Brexit.
IfG fellow and report author Akash Paun, said: “As with a dog walking on its hind legs, we should be impressed if the four governments manage to work together at all.
“But when it comes to Brexit, the stakes are high. If the dog topples over after a few tentative steps, and consensus cannot be reached, the result could be a constitutional crisis.”
May said she would offer the devolved administrations a forum “to allow them to help shape the UK's EU exit strategy”, in which they would have direct access to the secretary of state for exiting the EU, David Davis.
This would be a new sub-committee of the Joint Ministerial Committee of the four countries, which exists but meets rarely.
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.
“The country is facing a negotiation of tremendous importance and it is imperative that the devolved administrations play their part in making it work.”
She was also expected to say that, contrary to speculation, no final decisions have been taken and that how the UK leaves the EU will not boil down to a binary choice.
