A child of six could see the flaws in the Government’s legal argument in the Brexit case, the Supreme Court has been told.
The High Court ruled last month that Article 50 cannot be triggered without a Westminster vote.
On the fourth and final day of the legal challenge, the 11 judges heard from the Scottish and Welsh governments.
Richard Gordon QC said the Welsh Assembly was not trying to stall Brexit.
But he said Mrs May bypassing MPs to start Brexit would establish “a new constitutional order”.
He told the court “a child of six” could see the “flaw” in the Government’s legal case.
© Getty Businesswoman Gina Miller arrives at the Supreme Court for the final day of a four-day hearingThe 11 sitting justices were asked to consider whether the devolved governments of Scotland and Wales also need to be involved in the process.
Lord Advocate James Wolffe argued the Scottish Parliament was entitled to a voice on the issue, which involved the serious loss of EU rights for Scottish people.
He said it would be unlawful for the Article 50 process to start without a legislative consent motion from Holyrood.
Meanwhile, Manjit Gill QC, representing expats including children, said his clients faced being deported as a result of Brexit.
Without safeguards, he said, European Economic Area (EEA) nationals could be told “to pack your bags and go” when the UK withdraws from Europe.
He said the Government’s approach “drives a coach and horses” through the rights of those who have lawfully settled in the UK.
Attorney General Jeremy Wright QC said after the conclusion of the case that there were two things both sides agreed on: “Firstly, this is a case of great constitutional significance.
“Secondly, the people who brought this case in the first place were perfectly entitled to do it. The courts, now the Supreme Court, are perfectly entitled to decide it.”
Mr Wright defended the appeal, saying: “We don’t decide which cases are brought.
“The claimants bring their case, as they did.
© Reuters Pro-Europe demonstrators protest during a ‘March for Europe’ against the Brexit vote result in London, on 3 September”When we believe, as we did, that the judges weren’t right in the judgement they reached in the divisional court then the right thing is to appeal that judgement.
“That’s what we’ve done and we now wait and see what the Supreme Court has to say in the matter.”
The result of the case will not be known until January.
Last night MPs voted in favour of the Government’s timetable to trigger the formal process for leaving the European Union by March 2017.
:: Watch full coverage of day four of the case on Sky News, with a special programme on the final day of the historic challenge at 9.30pm.