The former UK diplomat who wrote Article 50, the legal process by which Britain will leave the EU, says Brexit could still be cancelled - even after exit negotiations begin.
John Kerr, who now sits as a cross-bench peer in the House of Lords, said the EU could not legally force Britain to leave even if it has already invoked Article 50 of the Lisbon Treaty.
It means Brexit could be stopped by another referendum or a parliamentary vote – even if exit talks are at an advanced stage.
Lord Kerr is the former head of the British diplomatic service and later became Secretary General of the European Convention, which drafted the precursor to the Lisbon Treaty that includes the Article 50 clause setting out the process for member states leaving the EU
The peer also backed calls for Parliament or the public to be given another vote that could stop Brexit.
Lord Kerr told the BBC: "It is not irrevocable - you can change your mind while the process is going on."
"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."
Theresa May has said she will trigger the two-year exit process by the end of March 2017, at which point some believe Brexit will be irreversible.
But Mr Kerr’s comments will give fresh hope to campaigners, including a number of cross-party MPs, who believe a second vote must be held before Britain leaves the EU.
The Government has repeatedly ruled out a second referendum or giving Parliament the option of overturning Brexit.
Lord Kerr is currently advising Scottish First Minister Nicola Sturgeon in her attempts to retain Scotland’s access to the EU single market even if the UK as a whole leaves.
The peer said he thought this scenario was unlikely to happen but that it might be possible to negotiate different arrangements for accessing the single market for different parts of the UK.
He said: "It is possible to envisage the Scots being in an ante room to the [European] council in Brussels, rather closer to the action than the English might be on particular subjects."
It comes as the High Court prepares to announce its decision on whether the government can invoke Article 50 without consulting Parliament.
The government believes Theresa May has the power to start the exit process unilaterally, but campaigners have argued that she must get MP’s approval.
A group, led by investment manager Gina Miller, has started legal proceedings, arguing that the 1972 European Communities Act cannot be overridden without a vote of Parliament.
The case is expected to be appealed to the Supreme Court, regardless of the verdict the High Court delivers.
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