Sir Konrad Schiemann even suggested such a scenario could be a “blessing in disguise” – but seemed to indicate an appeal could not delay the inevitable.
The judge, who previously sat at the Court of Justice of the European Union of which the ECJ is a part, suggested the legality of the Brexit deal could be challenged by a business or an individual – both at home or abroad.
And he warned such a challenge could be a “spanner in the works”, even resulting in “an extra year or maybe longer” to hammer out an improved deal.
Brexit has already been blighted by a series of legal challenges, with the Government facing months of resistance and costly legal battles.
Appeals and fresh challenges included ministers’ use of their executive powers to trigger Article 50 and on the Royal Prerogative.
Sir Konrad, who served on the CJEU between 2004 and 2012, said he could not see how any such challenge would stop Britain’s withdrawal.
And he did not know how likely an ECJ referral from a British court would be, only that it was a possibility.
He said: “I can see circumstances in which a company would want to bring through its national court, say France, a question to the European Court of Justice as to whether this agreement, which either has been concluded or is on the point of being concluded, is a lawful one.”
Sir Konrad said a delay could be a “blessing in disguise in the sense that it may permit all the parties on both sides to say ‘well of course we disapprove of any further delay but this is a spanner in the works which people didn’t anticipate, we have now got an extra year or maybe longer’.”
David Jones, the former Brexit minister, warned such a challenge could leave the UK in a “period of limbo” with “no certainty for business”.
And he claimed such a delay would simply be “kicking the can down the road” and delaying the inevitable exit from the Brussels bloc.
The row comes after the Government was forced to clarify its Brexit strategy after Tory rebels suggested Britain continue to accept the continued jurisdiction of the ECJ as regards to human rights after the UK’s departure,
Downing Street later stressed “the jurisdiction of the European Court of Justice will end” after a two year transition period.