The Liberal Democrat leader has said his party will vote against triggering Article 50 unless Theresa May promises a second referendum.
Tim Farron has said the public should have the chance to decide if they want to leave the European Union after the government's negotiations are complete.
MPs from other Parties have also said they are prepared to vote against initiating Brexit.
Mr Farron told BBC Radio 4's Today programme: "Article 50 would proceed but only if there is a referendum on the terms of the deal and if the British people are not respected then, yes, that is a red line and we would vote against the government."
In a Daily Telegraph online poll, 66% of people wanted a chance to vote on the outcome of Theresa May’s negotiations.
Tim Farron says party will oppose ‘deal being imposed on the British people that absolutely nobody voted for’
“It is the only logical and it is the only democratic option on the table. There will be a referendum at the end of this process so that nobody would have imposed upon them something they didn’t vote for.”
Farron said he respected the result of the June referendum, where 52% voted to leave the EU, but said it “must not now end up with a stitch-up, with a deal being imposed on the British people that absolutely nobody voted for”.