Brexit Secretary David Davis has said he does not know whether triggering Article 50 – the process from withdrawing from the European Union – can be reversed or not.
The government has always maintained that once the process to leave the EU was started, it could not be revoked.
However, Davis became the first cabinet minister to suggest that was not necessarily the case.
Speaking at a committee on Exiting the European Union for the first time, Davis said: "It is very, very difficult to see [Article 50] being revoked. We don't intend to revoke it, and it may not be revocable, I don't know."
The admission will give hope to political opponents who hope to pressure ministers into backing down on Article 50 after is has been triggered.
There is uncertainty about the situation because at the time of drafting of the Treaty on European Union, no one had expected the mechanism to be used.
However, Lord Kerr, the cross-bench peer who wrote the procedure believed it was "not irrevocable".
"You can change your mind while the process is going on," he told the BBC.
"During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time.
"They might try to extract a political price but legally they couldn't insist that you leave."
The cross-party committee, chaired by Labour MP Hilary Benn, also asked Davis about when Parliament can expect to receive the government's "plan" on Brexit, after a vote forced the Conservatives to reverse their policy of "not giving a running commentary".
Davis said: "It won't be next month. The policy work is still underway; there are quite a few decisions still to be made.
"We've carried out or are in the midst of carrying out about 57, I think, sectoral analyses, each of which has implications for individual parts of 85% of the economy, and some of those are still to be concluded.
"We have work still to be done on justice and home affairs. So there is a fair number of things still to do, so it will be as soon as we're ready."
Davis also said he would not be opposed to a transitional Brexit agreement, as long as the UK's general direction was confirmed first.
He said: "Whatever the transitional arrangement is, we need to know where we're going before we decide on the transition.
"If you build a bridge, you need to have both sides established before you build the bridge. So we need to know where we're going.
"It seems to me perfectly possible to know what the end game will be within two years."