Britain can rejoin the EU up until the end of March 2019, the French and German governments have said.
French President Emmanuel Macron said it was possible for the UK to decide to remain in the EU before the end of the two-year Brexit negotiations.
If Britain wants to rejoin the EU after that period it will have to apply for membership as an outside state.
Macron was speaking at a joint press conference with Prime Minister Theresa May.
Macron said he acknowledged that the British people had voted to leave and respected their decision.
His comments came after the German finance minister Wolfgang Schaeuble also said the UK could change its mind and remain in the EU.
Schaeuble told Bloomberg: “If they want to change their decision, of course they would find open doors, but I think it’s not very likely.”
After private talks with May in Paris, Macron said: “Of course the door remains open, always open until the Brexit negotiations come to an end.
“That being said, a sovereign decision was taken by the British people and that is to come out of the European Union, and I very much respect the decisions taken by the people, be it by the French people or the British people.
“In this case it’s not for me to say whether or not this decision should be questioned… but until the negotiations come to an end, of course there is always the possibility to re-open the door.
“But let us be clear and organised, and once the negotiations have started we should be well aware that it’ll be more difficult to move backwards.”
Talks between the UK and the EU are due to begin on June 19, however May’s hand has been drastically weakened since the general election she called to gain a “Brexit mandate” resulted in the Tories losing their majority.
The European Parliament’s Brexit coordinator, Guy Verhofstadt, also said the door to remaining in the EU would be open for the period of the negotiations, but Britain would lose the generous rebate on membership fees Margaret Thatcher negotiated in 1984.
Verhofstadt said: “Like Alice in Wonderland, not all the doors are the same. It will be a brand new door, with a new Europe, a Europe without rebates, without complexity, with real powers and with unity.”