British Prime Minister Theresa May insisted after triggering Article 50 of the Lisbon Treaty on Wednesday that it was a "historic moment from which there can be no turning back".
But hours later European Parliament President Antonio Tajani said the clock could in theory be reversed, if only with the approval of the other 27 member states.
"If the UK now changes its mind it can't do it alone, all the other member states of the union have to decide if they can do it or not," Italy's Tajani said.
The apparently contradictory remarks reflect a political and legal ambiguity about whether Britain is saying goodbye for ever.
The once obscure and now famous Article 50 is frustratingly vague on this issue, as on most others.
It says merely that "a Member State which decides to withdraw shall notify the European Council of its intention."
It then adds: "If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49," referring to the section for fresh applications to join the bloc.
Britain's Supreme Court said in January that it was "common ground" that the notice was "irrevocable and cannot be conditional", although it was not ruling on that subject but on parliament's supremacy.
The European Commission appears to be agree, but only on the surface.
"Once triggered, it cannot be unilaterally reversed. Notification is a point of no return," it said in a statement on Article 50 on Wednesday.
But this appears to leave open the possibility suggested by Tajani that the other 27 EU states could agree to let it reverse the notification.
'Can't be forced to leave'
The European Parliament mentions the possibility of revoking Article 50 in a resolution on Brexit that it intends to pass next week, if only to say that a "revocation of notification needs to be subject to conditions" so it cannot be "abused in an attempt to improve the current terms of the UK's membership".
A leading EU legal expert said that all the work on creating Article 50 nearly a decade ago had been focused on making it a smooth process for a country to exit, and that Brussels officials believed it was irreversible.
"But it was admitted that it could be considered that it is reversible in exceptional cases," added the expert, who spoke on condition of anonymity because of the sensitivity of his position.
The author of Article 50, John Kerr, who in a piece of historical irony is British, had the same idea.
"An Article 50 notification is not irrevocable," he told the House of Lords in February.
"If, having looked into the abyss, we were to change our minds about withdrawal, we certainly could and no one in Brussels could stop us."
Jean-Claude Piris, a former director of legal services at the European Council who is now a consultant, agreed.
"If the British authorities said 'I don't want to any more', you can't force a member state to leave," he told AFP. "My theory is that in this case you would return to the status quo."
'Quite punchy'
Ultimately, the question is a legal one.
Politically there appears to be little will in Britain at the moment to defy the result of a referendum that, however much of a shock, is uncontested.
But Jolyon Maugham, a British lawyer who has launched legal action in Ireland in a bid to force the European Court of Justice to rule on whether Britain can reverse Article 50, says that could still change.
"It is just quite punchy to assert that the political mood will remain static," he said.
"There is quite compelling evidence that people are changing their minds about the cost of Brexit for them."
If a decision ever does have to be made, it seems certain that it would be the ECJ that decides -- "even if one of the motivations for the British withdrawal is to secure its sovereignty", said a report by French MPs.
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