The British people are in danger of being "misled" by Government claims that Brexit cannot be reversed, the architect of Article 50 will warn.
Lord Kerr, the former UK ambassador to the European Union, will say during a speech in London later: "We can change our minds at any stage of the process."
Lord Kerr played a key role in drafting Article 50, the legal mechanism for a country to withdraw from the EU.
He will contradict the Government's view that the process is irreversible now that it has begun.
"We are not required to withdraw just because Mrs May sent her letter," the crossbench peer will say.
"The fact is that a political decision has been made, in this country, to maintain that there can be no going back.
"Actually, the country still has a free choice about whether to proceed. 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."
Brexit Secretary David Davis and the EU's chief negotiator Michel Barnier will meet in Brussels later on the second day of the latest round of talks before a crunch summit of European leaders next month.
Before his meeting with the Brexit Secretary, Mr Barnier said the moment was approaching for a "real clarification" of Britain's position on issues like citizens' rights, the Irish border and the UK's financial settlement.
If the EU27 agree next month that sufficient progress has been made on these issues, they will give a green light for negotiations to move on to the questions of trade and transition to a new post-Brexit negotiation.
Mr Barnier said close economic ties would depend on a "level playing field" and questioned whether the UK intended to break away from the European model of standards and regulation.
In a speech in Rome, he said: "Of course, the UK remains in Europe. But the British must tell us if they still adhere to the European model.
"Their answer is important because it will set the direction for the discussion of our future partnership and the conditions for its ratification."