The British people are in danger of being "misled" by Government claims that Brexit cannot be reversed, the architect of the Article 50 process is warning.
John Kerr, the former UK ambassador to the European Union, will insist that Theresa May's decision to send the letter triggering the Article 50 withdrawal process does not mean Brexit was inevitable.
"We can change our minds at any stage of the process," he will say as negotiations continue in Brussels on the UK's separation from the bloc.
Brexit Secretary David Davis and the EU's chief negotiator Michel Barnier will meet in Brussels on the second day of the latest round of talks before a crunch summit of European leaders next month which could decide whether to move on to the next stage of the process covering a future trade deal.
Kerr, who played a key role in drafting Article 50, the legal mechanism for a country to leave the EU, will use a speech in London to contradict the Government's view that the process is irreversible now that it has begun.
At an event hosted by the pro-EU Open Britain campaign the crossbench peer will say "we are not required to withdraw just because Mrs May sent her letter" to Brussels.
"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."
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."
Mr Barnier said the UK had to be clear about the consequences of leaving the EU and its single market and customs union.
"One cannot be half in and half out of the single market," he said. "One cannot end the free movement of people while holding on to the free movement of goods, services and capital.
"One cannot leave the single market and continue to enact its rules. One cannot leave the customs union but seek to benefit from frictionless trade with the EU.
"This single market is our principal economic asset.
"It is a body of laws, rules and norms chosen in common - and the UK knows them well since we have decided them together over 44 years - which we respect together, with common institutions and a common jurisdiction.
"There is no reason - none, I say - why the single market should be weakened by the departure of a member state."