The legal expert who drafted Article 50 has accused Theresa May of "misleading" the British public by claiming Brexit is irreversible, even if the date the country officially leaves the EU is written in the Withdrawal Bill.
Lord Kerr, the former UK ambassador to the European Union, was responding the government announcing they wanted to amend the bill to include a set date of 11pm (midnight in Brussels) on 29 March 2019.
Brexit secretary David Davis described the announcement of the exact time as an " important step [which] demonstrates our pragmatic approach to the vital piece of legislation".
Writing in The Telegraph, the prime minister warned that the government "will not tolerate attempts" to block the Brexit bill, in an apparent message to Remain Tory MPs planning on voting against it when it returns to Commons.
May added that the inclusion "in black and white" of the date Brexit will officially take place will ensure that the process cannot be blocked.
She wrote: "The British people have been clear. Parliament itself voted for Article 50 – and for this Bill at its Second Reading. We are leaving the European Union on 29th March 2019."
She added: "Let no-one doubt our determination or question our resolve, Brexit is happening.
"It will be there in black and white on the front page of this historic piece of legislation. The EU Withdrawal Bill is the single most significant piece of legislation in this Parliament because it is fundamental to delivering a smooth and orderly Brexit."
Speaking to BBC Radio 4's Today programme, Lord Kerr said that idea that Brexit must take place is a "political decision" and there will be no legislation meaning it has to continue, even in the date is included in the withdrawal bill.
He said: "The Brexiteers create the impression that is because of the way Article 50 is written that having sent in a letter on 29 March 2017 we must leave automatically on 29 March 2019 at the latest. That is not true.
"It is misleading to suggest that a decision that we are taking autonomously in this country about the timing of our departure, we are required to take by a provision of EU treaty law."
He added: "As far as the treaty is concerned there are lots of options. There is a provision to seek some extra time for negotiation and, much more important, there's the ability at any stage to take back the letter that the prime minister sent to President Tusk on 29 March.
"At any stage we can change our minds if we want to, and if we did we know that our partners would actually be very pleased indeed."
Keer, who is now a cross-party peer, made the comments ahead of a speech at an event hosted by the pro-EU Open Britain campaign in which he reiterated that the public "should not be misled" over claims that Brexit is irreversible now the process has begun.
He said: "While we're in, we're in. While the divorce talks proceed, the parties are still married. Reconciliation is still possible.
"The Article requires the parties to negotiate the arrangements for our withdrawal, but we are not required to withdraw just because Mrs May sent her letter.
"We can change our minds at any stage in the process."