The Lib Dem leader hit out at the Prime Minister for “not enacting the will of the people” in a furious clash with audience members on BBC Question Time.
He insisted he would not “give up” fighting a ‘hard’ Brexit as remaining a part of the single market had not been on the ballot paper.
He said: “If you lose, and occasionally I have, you should do two things. One is you should accept the result with good grace and the other, is you don’t give up.
Tim Farron said Theresa May was not "enacting the will of the people"
Theresa May is not enacting the will of the people
"And if you believe that, not my figures but the Conservative Government’s figures, we are, never mind a £50bn one off payment, which of course I would resist, you want to fight Britain’s corner and not have to pay that money, but the Government’s own figures say, never mind a £50bn one off, a £100bn extra a year debt because of the choice of a hard Brexit.
“And here’s the thing. Theresa May is not enacting the will of the people. If you are being generous, she is interpreting the will of the people.
“The single market was not on the ballot paper, maybe you wanted out of it, maybe you didn’t, none of us know because we weren’t asked.
In pictures: Theresa May meets with EU's Tusk Thu, April 6, 2017
The two leaders held talks on Brexit negotiations
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European Council President Donald Tusk gestures to members of the media as he leaves 10 Downing street after talks with British Prime Minister Theresa May in central London
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“That is why, if you are a democrat, you do not want to allow the politicians to rubber-stamp this stick up in two years’ time, you want the people to decide.”
Meanwhile, Nigel Farage ripped into the bloc’s chief Brexit negotiator Michel Barnier over a potential exit bill of more than £50bn.
The former Ukip leader claimed Brussels was planning to hold the UK to “ransom” despite 17.4million people voting to leave the EU last summer.