Theresa May has now signed a letter to trigger Article 50, the start of the official process to leave the EU.
Boris Johnson to renounce US citizenship
The London mayor has dual nationality as he was born in New York but has now revealed he wants to hand back his American passport.
Mr Johnson - who is running for parliament in Uxbridge and South Ruislip - said he would approach US ambassador Matthew Barzun about the change.
He said: "It's an accident of birth that has left me with this thing. I've got to find a way of sorting it out."
Constitutional experts believe dual nationality would not present any technical barrier to Mr Johnson becoming prime minister, but he would be likely to face questions about his loyalties.
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A man has been fined £1,800 for flying drones over Parliament, near Buckingham Palace and over football stadiums.
Government to investigate after some restaurants accused of withholding tips
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