A future trade deal between the UK and Brussels could be blocked by referendums in European Union countries, a former Foreign Office chief has indicated.

Lord Kerr, the architect of the Article 50 process for leaving the European Union, hit out at Prime Minister Theresa May and Foreign Secretary Boris Johnson as he claimed voters were in danger of being “misled” over Government claims that Brexit could not be reversed.

The former UK ambassador to the EU said he was “puzzled” by Government suggestions that a trade deal could be concluded by the time the UK leaves in March 2019 and added that any agreement could require ratification by the other 27 members, which in some cases would need public votes.

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Insisting that the Brexit process could be halted, he said: “We are not required to withdraw just because Mrs May sent her letter. We can change our minds at any stage during the process.”

He said a “political decision” has been made by Mrs May’s Government to maintain that “there can be no going back”.

But, he continued: “As new facts emerge, people are entitled to take a different view. And there’s nothing in Article 50 to stop them.

“I think the British people have the right to know this – they should not be misled.”

In a speech in London, he said Article 50 only covered the withdrawal process and any agreement about the future relationship on issues such as trade “would probably require ratification in every member state, which in some countries would require referendums”.

Countdown to Brexit, key events(PA graphic)

He said he was “puzzled by UK suggestions that a fully comprehensive agreement about the future can be completed and initialled by this time next year” in order to be signed off before the March 2019 withdrawal date.

Lord Kerr pointed out that the Canadian deal has taken seven years to negotiate and “got stuck in the Wallonian parliament”.

In a speech hosted by the pro-EU Open Britain group, he said it would be possible to extend the two-year Article 50 process and suggested other EU countries might agree to that if the UK wanted to hold a second referendum on whether or not to leave.

He said the EU found the UK “puzzlingly dogmatic and doctrinaire” and hit out at the Prime Minister’s decision to call a snap election after beginning the Article 50 countdown.

“I don’t know why Mrs May was in such a rush to send her letter in March, before her Cabinet had an agreed plan,” he said.

“It was odd to start the clock and not start negotiating, instead calling an election.”

He also hinted that Mr Johnson’s suggestion that Brussels could “go whistle” if it wanted a large divorce bill from the UK had entrenched the EU’s position on the timetable for the negotiations.

It was “unwise” for Brussels to insisted on “sufficient progress” being made on the divorce bill before beginning trade talks, he said.

“I think they were misled by suggestions here that they could ‘go whistle’ and that we might refuse to honour our commitments: I’m sure we never would.”

Lord Kerr said that once the UK was out of the EU, the only way back in would be through the accession process – and that the rebate secured by Margaret Thatcher would be gone, making it more expensive to rejoin than to remain in the bloc.