Sir Richard Aikens, who sat in the Court of Appeal from 2008 until 2015, spoke after the leader of the English Democrats political party launched a legal battle claiming that the UK has already left the EU.
Britain was supposed to leave on March 29, but was granted an extension by the EU until April 12 at a meeting last in Brussels last month.
But the English Democrats leader Robin Tilbrook, as well as number of Tory Brexiteers, says the new exit date should have gone before the House of Commons and the House of Lords before it was approved.
Mr Tilbrook yesterday announced he had submitted his case to the High Court and hoped to challenge the decision within weeks.
He says that, if his case is successful, the delayed leave date will be ‘null and void’ and Britain will already have left the EU and no longer be subject to its laws.
The case has been compared to the landmark ruling forced by campaigner Gina Miller, which led to judges ruling Parliament should have a vote on Brexit.
Respected former appeal judge Sir Richard Aikens has said the way in which the extension was organised is ‘highly unsatisfactory’ and ‘arguably illegal’.
Today, he told MailOnline the legal row over whether the government did act lawfully ‘can only be determined by a court.’
He added: ‘If the argument… is correct, then it would mean that, under UK law, we left the EU last Friday at 11pm. The Treaties would no longer be binding and the UK would no longer be subject to EU law.
‘The argument obviously becomes much more important if there is any attempt at a longer “extension”, but, logically, if the argument is correct, then any attempt at a further extension would be a legal nonsense as the UK would already be “out”.’
His comments will add weight to the claims that Britain may have accidentally dropped out of the EU and the ongoing wrangles over Theresa May’s withdrawal agreement are unnecessary.
Four Conservative MPs last week wrote to Theresa May also suggesting delaying Brexit in the way she did was illegal.
Sir Bill Cash, who was among the four, wrote: ‘We ask that you fully explain the actions taken by your Government and why you say that what it has done and is doing is legal.
‘We are gravely concerned that you are unlawfully seeking to extend the UK’s membership of the EU.’