Speaking at a conference, the Frenchman was asked what would happen if Britain changed its mind over Brexit.
He said: “If the UK changes its red lines, then we will adapt immediately.
“After it will be a third country, and like a third country it can ask to join the EU.”
Jacob Rees-Mogg was one of the first to respond to Mr Barnier’s comments.
He told The Sun: “It is unlikely the UK would want to rejoin because as a new member would be have to sign up to all the existing arrangements including the Euro and Schengen.”
Tory MP Nigel Evans said: “After seeing how abusive the EU has been during the negotiations, why would we ever want to?”
Mr Barnier also revealed former Ukip leader Nigel Farage told him the EU would collapse post-Brexit and vowed to oppose the Eurosceptic’s vision.
He told Mr Farage at a recent meeting: “You led Brexit, you won and now we are delivering Brexit.
“Can I ask you how you see the relationship between the UK and the EU after Brexit?”
Mr Farage said: “Mr Barnier, after Brexit the EU will no longer exist.”
The clash comes as Theresa May’s senior Cabinet ministers suggested they are aiming to secure a Brexit deal with the EU by the end of the month.
A leaked timetable, seen by the BBC, also reveals how ministers are set to give high-profile media interviews.
Downing Street dismissed the document saying the “childish language” shows it does not represent the Government’s Brexit strategy.
The UK is due to leave the EU on March 29, 2019 but both sides are yet to agree on the terms of Britain’s departure due to questions over what will happen to the Northern Ireland border being the main sticking point.