London (CNN) British Prime Minister Theresa May has ruled out a second referendum on Britain's exit from the European Union, saying it would be a "gross betrayal" of democracy.

Writing in the Sunday Telegraph , May said she would not "give in to those who want to re-open the whole question with a second referendum," adding that "to ask the question all over again would be a gross betrayal of our democracy -- and a betrayal of that trust."

Her comments came after "the People's Vote" campaign group, which is calling for a second Brexit referendum, recently secured a $1.3 million (£1 million) donation from British fashion mogul Julian Dunkerton.

UK voters in June 2016 voted yes in a referendum to leave the EU , more than four decades after joining. Brexit is due to take place on March 29, 2019, followed by a 21-month transition period when the divorce comes into effect.

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