Theresa May has been warned she could face a fresh court challenge over Brexit, after a retired Scottish doctor claimed the law requires there to be a second referendum on any withdrawal agreement.

Dr Andrew Watt, a former radiologist from Glasgow, sent a letter before action – usually the first step in taking disputes to court – to the Prime Minister, in which he argued Section 2 of the European Union Act 2011 requires a referendum before the UK can leave the EU.

He said the act, which was designed to trigger a referendum in the event of a new EU treaty that would transfer powers to the bloc from the UK, calls for the Government to have a referendum before it can ratify any treaty that amends or replaces the EU or the functioning of the bloc.

Dr Watt highlighted a section of the legislation which sets out that there must be a referendum if there is the “conferring on an EU institution or body of power to impose a requirement or obligation on the United Kingdom, or the removal of any limitation on any such power of an EU institution or body”.

He added that a separate line – providing for a referendum if the EU is given the power to impose sanctions on the UK – could also apply if the bloc is able to erect tariffs on UK goods and services.

Dr Watt said: “The Prime Minister has refused to hold a further referendum on withdrawal from the European Union. She has overlooked the fact that Section 2 of the European Union Act 2011 requires a referendum before the United Kingdom can ratify any withdrawal or related trade agreement with the European Union.

“The Prime Minister’s failure to recognise the legal requirement for a further referendum means that she is at high risk of running out of time in the negotiations with the European Union.”

He said he has asked Ms May to respond formally to his letter by Friday 12 May.

“Once I have her response I can decide which of the legal issues in the letter before action should be pursued in the High Court,” Dr Watt added.

“At that time I expect to seek to raise funds by crowdfunding or other means to enable this important legal action to go ahead.”

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It comes three months after the Government lost a court challenge brought by philanthropist and banker Gina Miller, which meant it had to bring a bill to start the legal Brexit process to Parliament rather than launch it unilaterally, which Ms May had initially set out to do.

In the most recent Brexit development, the EU Council President Donald Tusk said at a special summit in Brussels that the bloc’s Brexit negotiation guidelines have been unanimously agreed.

The Prime Minister meanwhile accused EU leaders of preparing to “line up to oppose us” over Brexit, as she made an audacious appeal to Labour supporters to “lend me their vote” to strengthen her hand.