Tim Farron has claimed the Liberal Democrats can replace Labour as Britain's main opposition party because Jeremy Corbyn is "demonstrably the worst leader in British political history".

Despite opinion polls showing the Lib Dems lagging far behind Labour on around 10 per cent, Farron called on voters who want a strong opposition to back his party.

He drew attention to the Labour leader's perceived weakness on security issues, insisting politics needs an opposition "that is proud of its country, that is prepared to defend its country and prepared to defend public services".

"I want to be the leader of the opposition," Farron said in an interview with the i newspaper.

"Jeremy Corbyn is a perfectly nice man, but is demonstrably the worst leader in British political history in terms of effectiveness.

"As things stand, a feeble Labour opposition will mean the Conservatives will have untrammelled power.

"Britain needs a decent, strong opposition and my job is to ask the British people to give that job to me."

He also urged voters in Scotland, where the Tories are currently the leading unionist party with the Lib Dems trailing in fourth according to polls, to back his party.

"Scotland desperately needs a left-of-centre progressive unionist party," he said.

"The idea people who believe in keeping the United Kingdom together have to vote Conservative is unpalatable."