Dr Andrew Watt, a retired Scottish doctor, said Ms May’s notification to the head of the European Council Donald Tusk of the UK’s intention to leave the EU on 29 March is void because the country never officially decided to leave the union.
He argued last year’s EU referendum was not a legally binding decision whereas according to Article 50 of the Lisbon Treaty, member states have to decide to leave the EU in accordance with relevant constitutional requirements before notifying Brussels of their intention.
Dr Watt said the UK never took the constitutional decision to leave the EU, making unlawful any later notice of withdrawal.
The appeal comes months after business owner Gina Miller won a legal challenge against the Secretary of State for Exiting the European Union, which concluded the Government had to bring a bill before Parliament to begin the Brexit process.
In the Supreme Court ruling on the case, it was made clear the EU referendum on 23 June was not a decision, but only a notice of opinion from the British public on which Parliament then had to act.
The court stated: “Both the Secretary of State and the Miller claimants proceed on the basis that the referendum result was not itself a decision by the UK to withdraw from the EU, in accordance with the UK’s constitutional requirements.”
In the Government’s own submission to the Supreme Court, it accepted the high court’s previous judgement, which was upheld, and stated: “If the UK is to withdraw from the EU, Parliament must be asked to answer precisely the same question which was put by Parliament to the electorate and has been answered in the referendum, and must give the same answer in legislative form.”
Yet responding subsequently to a Freedom of Information Request, the Department for Exiting the European Union stated in March the “decision” on leaving the EU was made in the referendum on 23 June.
Since Parliament only voted on a bill to notify Brussels of the UK’s intention to withdraw from the EU, Dr Watt argues there was no lawful decision to leave the union, making the Article 50 notification letter unlawful.
Speaking to The Independent, Dr Watt said: "If, as I believe, Theresa May has misled the public, the Westminster Parliament and the European Union about a UK exit, it seems to me that she can expect to be in very severe political difficulty indeed. Her U-turn on social care will seem trivial by comparison.”
He is now seeking a judicial review on the issue and has written to the European Commission in the hope Mr Tusk, who received Ms May’s letter formally notifying of the UK’s intention to leave the EU, would take part as an interested party.
In the letter, seen by The Independent and sent to Christine Dalby, the acting head of representation of the European Commission in London, Dr Watt sets out the aim of a prospective application for a judicial review.
He urged the European Commission to make Mr Tusk aware of his application “as a matter of urgency” and described the resulting legal and political considerations as “of enormous importance”.
He also called on the European Parliament and the 27 members of the EU to be made aware of the issue.
Dr Watt said: "Donald Tusk, as President of the European Council, has a duty to ensure that everything done that relates to a UK exit or 'Brexit' is done in accordance with law. He is an ideal person to be an interested party in the prospective judicial review.
The retired doctor added he believed he has “a good chance of success”.
The administrative court will have to grant permission for the judicial review to go ahead once the application is made.
Last month, Dr Watt sent a letter before action to the Government’s Legal Department informing it of his intention to seek a judicial review.
Oliver Gilman, a senior lawyer at the Government’s Legal Department, responded to the letter and said the argument set out by Dr Watt was “without merit” and that the European Union (Notification of Withdrawal) Act 2017 satisfied the requirement for a valid notice.
A government spokeswoman said: "The British people voted to leave the EU, and the government will deliver on their verdict.
"It’s important to remember that Parliament backed the referendum by a margin of six to one and has already indicated its support for getting on with the process of exit to the timetable we have set out."
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies