THERESA May “alienated” the Scottish and Welsh governments by breaking promises over Brexit and her successor cannot afford to make the same mistake, a new report warns.

The Institute for Government (IFG) said the new Prime Minister must secure the support of the devolved nations when the UK enters the next, far more complex phase of Brexit.

The think tank said engaging with a pro-independence government in Edinburgh was always going to be “challenging” for Number 10.

However that did “not excuse the failure to follow through on the Prime Minister’s early promises”, or the “secretive and defensive” way the UK government operated.

It said: “The Prime Minister made bold promises to the Scottish First Minister on her first visit to Edinburgh in July 2016, which her Government then notably failed to follow through. The UK Government needs to bring the devolved administrations back on board as ‘fully engaged’ partners in the Brexit process, and do so as soon as possible.”

In a brutal assessment of Mrs May’s handling of the process so far, the IFG catalogued a series of errors made at the start of her premiership.

Early negotiations were “bedevilled” by cabinet disagreements over whether to go for a soft or hard Brexit, with the PM focused too much on preserving party unity.

Splitting responsibilities between Downing Street and the Department for Exiting the European Union (DExEU) was also “misguided”, as it led to tensions between the two.

“This feeling of division at the top of the negotiating team was compounded by the secretive approach adopted by the Prime Minister and her advisers,” it said.

The UK Government also failed to engage with parliament, only sharing key information with MPs after being held in contempt, and resisting initial demands for a meaningful vote.

It “alienated the devolved governments”, culminating in a first for devolution: Holyrood withholding consent for the main Brexit law at Westminster and then being overruled.

It said: “This should not become the norm. Early and regular engagement with the devolved governments and legislatures should help avoid further damage to intra-governmental relations.”

The government also tried a ‘divide and rule’ strategy of woo individual EU leaders to back its position that completely backfired, and led the EU27 to present a more united front.

“The UK’s attempts to divide and conquer were not helped by tone deaf interventions by ministers and senior politicians designed to appeal to domestic audiences,” it said.

When the UK was successful, such as in talks on the European Atomic Energy Community, it was because the UK decided on its objectives early and engaged with specifics.

The report warned the UK similarly risks “stumbling” into talks about future trade and security relations, potentially putting the country at a disadvantage for decades.

It said a clear plan and a focus on detail was the key to the next phase, and recommended the Prime Minister appoint a political deputy to “knock departmental heads together”.

The Cabinet Office, not DExEU, should co-ordinate the negotiations, and the under-used experts at the Department for International Trade should be at the heart of the talks.

“The EU made a virtue of its transparency and willingness to engage during Phase One, whereas the UK was secretive and defensive, and failed to engage properly with Parliament, the devolved administrations, business groups and civil society,” it said.

If the Government was serious about negotiating a future relationship that worked for the whole UK, it had to engage with the devolved governments and accept outside input.

To improve transparency, a designated select committee should hold regular hearings on the negotiations and engage with Holyrood committees.

Through the existing Joint Ministerial Committee, the Prime Minister “should discuss any mandate for the future relationship with the devolved First Ministers, giving them an opportunity to voice their thoughts and concerns – but not to veto it”.

Holyrood and the Welsh Assembly should also get regular updates, but not a vote on the final deal, it said.

UK ministers should be willing to give evidence on progress to committees of the devolved legislatures.

IFG programme director Jill Rutter said: “Hasty decisions created completely foreseeable problems for the exit negotiations, compounded by the inability of the Cabinet to reach an agreed position on the key future economic relationship.

“Whoever is Prime Minister for the second phase of the negotiations needs to ensure that they avoid similar mistakes next time round.”

SNP Brexit Secretary Michael Russell said the report backed up the Scottish Government’s complaints about Scotland being “ignored”.

He said: “This report merely confirms what we have consistently said – namely, that Scotland has been ignored by the UK Government throughout the Brexit process with scant, if any, notice taken of our overwhelming vote to remain in the EU.

“The Scottish Government’s detailed compromise proposals have also been ignored or dismissed at every turn.

“This report makes clear that that approach must now change. With the Brexit deadline now extended until 31 October, that extra time must be used wisely, allowing for all parties and all devolved governments to be properly consulted and involved. The Prime Minister must drop her red lines, which have led to the Brexit impasse.”

The UK Government said it had “worked constructively” with the devolved administrations throughout the first phase of negotiations in order to deliver a Brexit that works for Scotland, Wales, England and Northern Ireland.

A spokesperson said: “There are already regular discussions between the UK and the Devolved Administrations at all levels and the Prime Minister has set out that there will be an enhanced role for them in the next phase.”