The former right-hand man to Theresa May admits it was wrong to enter talks about Britain’s divorce from the European Union before starting trade negotiations.

And Gavin Barwell said he is “sceptical” that her successor Boris Johnson will be able to do any better when it comes to renegotiating the backstop.

Former Brexit secretary David Davis had originally said he wanted to talk about the UK’s exit from the Brussels bloc at the same time as trade discussions commenced.

But on June 20 – 10 days after former MP Gavin Barwell took the job as Mrs May’s chief of staff – the Government conceded to EU demands to thrash out the divorce bill, the Northern Irish border and the future of EU citizens living in Brexit before all else.

Mr Barwell, who was controversially awarded a position in the House of Lords in his former boss’ resignation honours, told BBC Radio 5 Live that it was “obviously” wrong to buckle and blamed the decision to adhere to the EU’s timetable for the current stalemate over the backstop.

The Northern Irish backstop, agreed by the former prime minister and the EU, agreed to keep the UK indefinitely in a customs union with Brussels if no Brexit deal could be agreed that did not involve border checks in Ireland.

When the ex-Croydon Central MP was asked what went awry during the negotiations, he said: “One of the things that I would reflect on is the original phasing decision, which was something the EU pushed for very hard. But that definitely, with hindsight, made it harder.

“That split is what essentially led to the backstop because at the point that you’re having your divorce, you haven’t settled what your relationship is going to be like in the future.

“The EU argued that – both to protect their single market, and also to keep the border in Ireland as it is today – you needed some kind of insurance policy.

Barwell May
Gavin Barwell pictured with his former boss, Theresa May (Neil Hall/PA)

“And that wouldn’t have been necessary if we’d done both sets of talks at the same time. If I was to identify a single issue, that’s probably the one that I would pick.”

He said the EU was also to blame for the decision to pursue divorce terms before any future relationship had been agreed.

“It takes two people to agree to something, but if we hadn’t agreed to that, we wouldn’t have been able to make progress in the negotiations,” said Mr Barwell.

“And I think, from my conversations, particularly in the last few months in the job, there are quite a few people on the EU side who think they were wrong to push that.”

Prime Minister Mr Johnson continues to stand by his commitment to rid the Withdrawal Agreement of the backstop “whole and entire” before agreeing to an exit deal.

But Mr Barwell said he was not hopeful of such a strategy working well for the current Conservative Party leader.

“I’m obviously sceptical because we tried a number of these different alternatives when we were in government and weren’t able to get them through,” said the former MP of seven years.

Having lost his seat in the snap 2017 election, former minister Mr Barwell had a new job in Downing Street only days later as Mrs May looked to re-model her inner circle and regain her authority after losing her Commons majority.

General Election 2017 aftermath
Gavin Barwell replaced former advisers Nick Timothy and Fiona Hill at Number 10 (Rick Findler/PA)

She sacked former advisors Nick Timothy and Fiona Hill following the hung parliament result and brought Mr Barwell, a well-liked Tory who had experience of winning a marginal seat, into the fold.

Mr Barwell, understood to have preferred a consensual approach to securing a Brexit deal while in post, admitted the government should have started cross-party talks much earlier in the proceedings.

It was only after Mrs May had lost the third vote on her Brexit deal in March that senior Opposition figures were brought in to see whether there was a deal the Commons could support. Cross-party talks lasted two months before collapsing in May.

“If I reflect back on it, probably with hindsight, I might have started that cross party process a bit earlier,” he told The Emma Barnett Show.

Mr Barwell is soon to be a Lord after being offered a life peerage in Mrs May’s resignation offers.

The Londoner said he recognised that some critics saw it as a “reward for failure” but argued he had contributions to make on issues such as housing in Parliament’s second chamber.