British Prime Minister Theresa May should stop misleading voters and admit that Brexit can be avoided if Britain decides unilaterally to scrap divorce talks, the man who drafted Article 50 of the Lisbon Treaty said.
May formally notified the European Union of Britain's intention to leave the EU by triggering Article 50 of the treaty on March 29.
She said she would not tolerate any attempt in parliament to block Brexit.
By triggering Article 50, May set the clock ticking on a two-year exit process that has so far failed to yield a divorce deal and which was interrupted by her gamble on a snap election in June which cost her party its majority in parliament.
"While the divorce talks proceed, the parties are still married. Reconciliation is still possible," John Kerr, British ambassador to the EU from 1990 to 1995, said in a speech in London.
"We can change our minds at any stage during the process," said Kerr, who added that the legalities of Article 50 had been misrepresented in Britain.
"The British people have the right to know this: they shouldn't be misled," he added.
The day May triggered Article 50, she told the British parliament that there was "no turning back" and today insisted that the UK would be leaving the EU at 2300 GMT on March 29 2019.
In a June 2016 referendum, 51.9% of voters backed leaving the EU while 48.1% voted to remain.
Brexit supporters argue any attempt to halt the exit process would be anti-democratic, while opponents say the country should have a right to pass final judgement on any exit deal negotiated.
May, an initial opponent of Brexit who won the top job in the political turmoil that followed the vote, said last month that Britain would not revoke Article 50.
But ever since the referendum, opponents of Britain's exit - from French President Emmanuel Macron and former British prime minister Tony Blair to billionaire investor George Soros - have suggested Britain could change its mind and avoid what they say will be disastrous consequences for the British economy.
There are few signs of a change of heart on Brexit in opinion polls so far.
Both May's Conservatives and the opposition Labour Party now explicitly support leaving the EU, which Britain joined in 1973.
Supporters of Brexit have repeatedly said that any attempt to have another referendum, or to undermine Brexit, would catapult the world's fifth largest economy into crisis.
But the Brexit process has been challenged in a number of cases in British courts, many focusing on the as-yet unanswered question: Can Article 50 be reversed?
The 256-word clause does not say whether it can be revoked once it is invoked.
This means that, if lawyers ask for clarification, the question would have to go to the European Court of Justice, the EU's highest court.
Kerr, who in 2002-2003 acted as secretary-general of the European Constitutional Convention that drafted Article 50, said the debate had been misrepresented inside Britain.
It was clear, he said, that May's Article 50 letter could be revoked.
Such is the interest in the legalities of Brexit that one prominent lawyer, Jessica Simor, has formally asked for May's unpublished legal advice on the matter.
"Britain can basically change its mind at any time right up to the 29th of March 2019," Simor told Reuters last month.
"If you can revoke Article 50, then parliament has the power to rescue the country if that becomes necessary - if the government fails to secure a deal, or the deal is terrible, or the people do not want it," she added.