LONDON (Reuters) - The leader of Britain's Liberal Democrat Party said on Friday his lawmakers would vote against triggering the formal process of leaving the European Union unless the government promises to hold a second referendum.

Tim Farron, whose party has only have eight members in parliament's lower House of Commons, has long called for a second referendum on the terms of any deal with EU leaders.

He said his party would now vote against triggering Article 50 of the Lisbon Treaty which kicks off the EU divorce process, if its demand was ignored.

"We'd vote against Article 50 if our 'red line' is not met ... which is we want to respect the will of the people and that means they must have their say on a referendum on the terms of the deal," he told BBC radio.

British Prime Minister Theresa May has said she will invoke Article 50 before the end of next March but last week London's High Court ruled that this would require the approval of parliament, a step which could potentially delay or even derail her plans.

May has still promised to deliver a full exit from the EU and the government are appealing the legal decision to the Supreme Court, the UK's highest judicial body which will hear the case next month.

In June, Britons voted in a referendum to leave the EU by 52 percent to 48 percent but Farron said a second referendum was critical "so that nobody would have imposed upon them something they didn't vote for."

"We believe that what started with democracy last June, which we totally respect, must not now end up with a stitch up, with a deal being imposed on the British people that absolutely nobody voted for," said Farron.

Several lawmakers (MPs) from Britain's opposition Labour party have also said they might vote against triggering Article 50, though the party's leadership has indicated it would not seek to block the process.

"Because they didn't like the first answer, Liberal Democrat and Labour MPs seek to put the question all over again in hope of a different answer," said junior Brexit minister David Jones.

(Reporting by James Davey; editing by Michael Holden)