The UK will be given until the end of 2020 to change its mind on leaving the single market and customs union, the EU’s chief negotiator has said.

Michel Barnier said the UK government has the full length of the Brexit transition period to reverse its decision to leave the two trading blocs.

The EU would be willing to amend its “red lines” if Britain does the same, he said, adding that there would be “no dogmatism” from Brussels.

The confirmation leaves open the option of the UK remaining part of EU institutions if striking new trade deals with other countries proves more difficult than predicted.

It will come as a boost to pro-EU campaigners who are urging the government to soften its Brexit stance.

The Brexit transition period is due to end on 31 December 2020 – 21 months after Britain formally leaves the EU on 29 March 2019.

What is the Brexit 'transition'?

The UK will remain part of the single market and customs union until then, but Theresa May has insisted it will leave both, as well as the jurisdiction of the European Court of Justice, when the transition period ends.

Mr Barnier told reporters: "So long as they have not left, during the transition period, everything is still possible.

"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."

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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."

The EU said last month that it is prepared to shift its position if the UK backs down over its plans for a hard Brexit.

Guidelines published by the European Council ahead of the second round of Brexit talks said their position was “compatible with the positions stated by the UK” but added: "If these positions were to evolve, the Union will be prepared to reconsider its offer."

Mr Barnier’s confirmation was welcomed by anti-Brexit campaigners.

Labour MP Chris Leslie, a supporter of the Open Britain group, said: "These comments by Michel Barnier demonstrate yet again how ludicrous it was for the Government to take single market membership off the table in the Brexit negotiations.

"Leaving the single market will lead to huge economic damage, years of austerity and will starve our public services such as the NHS of vital resources.