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Britain has until the end of 2020 to reverse its position on leaving the single market and customs union, Michel Barnier has said.

The European Union’s chief negotiator said if the UK changed its “red lines” during the post-Brexit transition period then the EU would follow suit.

It comes after Prime Minister Theresa May said Britain will quit the single market and customs union.

The EU official gave Mrs May until December 31, 2020 to change her mind on Britain exiting the pair as it withdraws from the EU.

He said: "So long as they have not left, during the transition period, everything is still possible."

The Prime Minister used a speech in March to repeat her "red lines" that the UK will leave the single market and customs union as well as the jurisdiction of the European Court of Justice.

The UK will formally leave the EU on March 29 2019, two years after the invocation of Article 50.

The 21-month transition period is due to end on December 31 the following year, coinciding with the end of the EU's seven-year budget.

Speaking to European reporters, Mr Barnier said: "If the British wish to modify their red lines, we will modify ours in consequence.

"I am not hearing that today but everything is possible, there is no dogmatism."

He added: "What creates the problem in Ireland, is the decision of the United Kingdom to leave the EU but also to leave what it is not obligatory to leave, that's to say the single market and the customs union."

