The portrait of the prime minister notifying Brussels of her decision to trigger Article 50, which formally began the two-year countdown to Britain’s departure from the EU, was given to Ms May by Tory peer Lord Moynihan, according to the official register.
Painted by artist James Drake, it depicts Ms May seated at a desk in Downing Street, penning the historic missive to European Council president Donald Tusk last year.
The painting was valued at £250, meaning Ms May had to pay £110 to keep the painting so she could hang it in her private quarters, according to Downing Street.
Under the government’s ethics code, there is an £140 limit on gifts before ministers have to pay to keep them.
Lord Moynihan, a former Olympic rower and ex-minister, arranged for the picture to be presented to Ms May, who attended Oxford University with him in the 1970s.
He told the Daily Telegraph that the painting was a "remarkable" work which demonstrated "strength, empathy and respect" for the prime minister.
Lord Moynihan said "James Drake is a close friend. He approached me to say he had painted the Prime Minister and asked me to ensure she received the painting as a gift.
"I understand she very much likes the way in which he has captured this historic moment.”
Ms May signed the six-page letter triggering Article 50 on March 28 last year, before it was taken to Brussels on March 29 and handed to Sir Tim Barrow, the UK's ambassador to the EU, to deliver to Mr Tusk.
Later, it emerged that the historic letter had travelled to Brussels in first class on a Eurostar train, accompanied by two civil servants.
The total cost of delivering the letter was £985.50, including a £1.50 per person booking fee, according to a freedom of information request to the Foreign Office.
The letter did not get its own seat.
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