SIR Vince Cable will today unveil radical plans to widen the Liberal Democrats’ membership with a new category of “supporter” in an attempt to make the party a “movement for moderates” and the home for anti-Brexiteers.

However, the keynote speech about the Lib Dems’ future is likely to prompt questions about the longevity of Sir Vince’s own leadership given that within days of the next General Election, due in 2022, he will be entering his 80th year.

Party sources have told The Herald that there is a growing expectation that the deputy leader Jo Swinson, 38, will take Sir Vince’s place within 12 months, barring a snap poll in the wake of Brexit.

Already talk of introducing a Canadian-style plan to open up the leadership to a non-MP appears to have rapidly quietened. One senior party figure branded the proposal “bonkers”.

In the speech, at the National Liberal Club in London, Sir Vince will propose the supporters’ scheme to get what were described as “moderate, liberal-minded voters” involved in campaigns such as opposing Brexit.

He will explain that the proposal would mean the new class of supporters would pay nothing to sign up to the party’s values yet enjoy a range of entitlements, including the right to vote for the leadership and to shape the party’s campaigning online.

“The Liberal Democrats already have an army of voluntary helpers and deliverers, as well as 200,000 online supporters, who loosely identify with us and campaign with us but, currently, have no say in the direction of the party.

“Whatever rights our new supporters gain, we as a party aim to be in constant conversation with them, engaging them in campaigns and urging them to begin campaigns of their own. I want these not to be just about stopping things but about growing support for the things that matter to Liberal Democrat voters and to the vast swathe of voters in the centre ground whom we are yet to persuade,” Sir Vince is due to say.

He will note that groups like More United, 38 Degrees, Avaaz and Change.org have shown how such regular conversations can happen and help engage hundreds of thousands of people online.

“I want our party to do that and to offer our movement a political arm within Parliament. So it is not just a protest group banging at the door but a movement with a voice on the inside; our parliamentary party.”

The party leader will add: “The Liberal Democrats are not a socialist party concerned with extreme-left entryism or a right-wing party trying to keep out extreme right-wingers. We are a centre-ground, pro-European, liberal and social democratic party, welcoming like-minded supporters. This will be a Movement for Moderates.”