The claim: Prime Minister Theresa May says there is no turning back from the triggering of Article 50, which starts the process of leaving the EU.
Reality Check verdict: The government is clear that it respects the result of the referendum, so it argues that any debate is theoretical. However, the question of whether Article 50 is irrevocable is the subject of legal dispute.
As a result, the UK government has been unable to make any definitive legal statements on the issue.
The Justice Secretary, Liz Truss, has said "My understanding is that it is irrevocable," while the Brexit Secretary, David Davis, said in December 2016: "Is it irrevocable? I don't know."
In the recent UK Supreme Court case on Article 50, both sides assumed that it was irrevocable and the court judgement concluded: "We are content to proceed on the basis that that is correct, without expressing any view of our own."
So this is not an issue that has been formally tested in a UK court.
But European Council President Donald Tusk has said that he believes Article 50 can be reversed.
When asked if the UK could unilaterally withdraw its Article 50 notification during the next two years, he said, "Formally, legally, yes."
Lord Kerr - the former British ambassador to the EU, who helped draft Article 50 - agreed.
"You can change your mind while the process is going on," he said.
He acknowledged that this might annoy the rest of the EU, and be seen as a huge waste of time.
"They might try to extract a political price," Lord Kerr said, "but legally they couldn't insist that you leave."
The distinction between political and legal opinion on this issue is critical. The politics might become way too complicated if the UK tried to change its mind.
But who might have the final legal say on what could yet become a critical question?
Article 50 is a piece of European law, so the ultimate arbiter on this issue is the European Court of Justice.
There is an ongoing case in Dublin at the moment that is seeking to refer the question of irrevocability to the European Court to get a definitive answer.
One other point is worth bearing in mind: everyone is talking about a two-year period for negotiating under Article 50, at the end of which the UK would leave the EU.
But Article 50 does provide for that two-year period to be extended, if all 28 EU countries, including the UK, agreed.
No-one is advocating that, but it remains a legal possibility.
The argument that Article 50 cannot be reversed once it has been triggered has not been tested in court.
The rest of the EU has said it does not want the UK to leave, but - politically speaking - it would be very difficult to revoke notification of Article 50, and the current UK government says it has no intention of doing so.
Read more from Reality Check

Tory PCC Adam Simmonds' plans include employing a chief of staff, four assistant commissioners, and a value for money officer.
Under the old police authority, there were seven staff members. The new organisation will have 10 extra posts.
Mr Simmonds said a team was vital in overseeing the county force.
He said: "This is not an empire. This is about building something which is brand new. No-one has been PCC for Northamptonshire before.
"The public voted for me to do a job to support frontline policing. I can't do it physically on my own."
He said his plans had been put forward to the public for consultation.
Mr Simmonds said he would be judged on whether he had achieved his objectives of more police and a reduction in drug crime.
In a report Mr Simmonds said he hoped to create three new departments, for drugs eradication, faith-based and neighbourhood initiatives, and the protection and security of the county, would also be created.
The commission will carry out the work that the previous police authority did of overseeing targets and setting budgets.
But the office also has new responsibilities of prevention and taking care of victims of crime.
For 2012-13, the budget for Northamptonshire Police is Â£120.55m - Â£2m less than 2011-12.
Savings of Â£20m have to be made over four years.
A spokesman for the commissioner said staff would be consulted about the plans in the first four months of next year.
Article 50 of the Lisbon Treaty was written with a deliberate lack of clarity - it does not say whether it can be revoked once it has been triggered.
[NEXT_CONCEPT]
The new police and crime commissioner (PCC) for Northamptonshire has denied he is "empire building" with his plans for a 17-strong team.