Britain's representative to the European Union Tim Barrow hand delivers Theresa May's Brexit letter in notice of the UK's intention to leave the bloc to EU Council President Donald Tusk: Reuters

Theresa May has been accused of sending an "unlawful" letter of notification to Brussels over the UK's intention to leave the EU, and is now facing a judicial review.

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 but according to Article 50 of the Lisbon Treaty, member states have to decide to leave the EU in accordance with 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 Existing the European Union, which concluded the Government had to bring a bill to Parliament to start the legal Brexit process.

In the Supreme Court ruling of 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 agree.

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.”

View photos Donald Tusk showing the letter he received signed by Prime Minister Theresa May, which formally notified of the UK's intention to exit the EU (Reuters) More

If the Secretary of State accepted in Ms Miller'scase the referendum was not a decision to leave the EU, the Government allegedly acted otherwise.

Responding 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 about to launch 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 “of enormous importance”.

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