Tim Farron says the Liberal Democrats will be putting the UK’s membership of the EU single market “front and centre” of their general election campaign.
Mr Farron said the 8 June election that Theresa May has called for was a “chance to change the direction of our country”.
He said his party would “avoid a disastrous hard Brexit”.
MPs will vote on Wednesday on whether to approve Mrs May’s call for an early general election.
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The Lib Dems will be looking to recover ground lost in the 2015 general election, when they were left with just eight MPs.
The party has since gained a Parliamentary seat from the Conservatives in Richmond in London and is hoping to tempt pro-Remain supporters with a promise of a referendum on the terms of the Brexit deal.
Mr Farron claimed his party was the only one capable of prevent an overall Conservative majority.
He said his party would fight to stay in the single market, which Mrs May has said the UK will be leaving when it leaves the EU.
General Election: Tim Farron vows to fight ‘hard Brexit’}