Sir Vince Cable will step down as leader of the Liberal Democrats next month.

In an interview with Business Insider, he said he was "optimistic" that more MPs will follow Chuka Umunna and join the Lib Dems in the coming weeks: "There is every prospect that they will come across."

Cable predicted a "big and substantial breakthrough" for the party at the next general election after it gained over 700 seats at the local elections and finished second in the European elections.

The Lib Dems are ready for an Autumn election and could win another 60 seats, he said.

Either Jo Swinson or Ed Davey will be chosen as Cable's successor next month.

LONDON — Sir Vince Cable will go out on a high when he steps down as Liberal Democrat leader next month after recently leading the party to over 700 gains local elections and a historic second place finish in the European elections.

As he prepares to hand over the party leadership to one of two successors, polling suggests the resurgent Lib Dems are level with both the major parties in British politics — thanks to a swell in support from Remain supporters.

The Lib Dems are also favourites to win the upcoming Brecon and Radnorshire by-election from the Conservatives.

A Lib Dem victory would leave the Conservative government with a working majority in House of Commons of just three, and the United Kingdom on the edge of another snap general election.

Cable told Business Insider this week that he expected a "big and substantial breakthrough" for the Lib Dems at the next general election which could result in his successor — Jo Swinson or Ed Davey — leading over seventy MPs.

"I was a bit nervous about the idea [of an early general election] before all of this happened," he said.

"If there had been a no confidence vote, we would have voted for it.

"But we weren't at that stage ready. We are now ready."

Cable added: "We have thirty target seats at the moment and we can probably without an enormous excursion, double that. If an election comes — in late Autumn perhaps — we'll be in a very good position to fight it."

The outgoing leader said that the acquisition of "major political figure" Chuka Umunna had triggered a "sudden spurt" of new members, with around 800 joining in the hours after he was unveiled as the party's twelfth MP.

Umunna joined the Lib Dems last week having quit Labour to launch new centrist party Change UK in February.

"People quite like this idea of people in the middle-ground working together rather than fighting with each other, whether it's alliances or joining the party. It sends the right signals," Cable said.

On Umunna's claim in an interview with Business Insider that he'd be a Lib Dem until the day he died, Cable joked: "Well I don't know exactly what I'll be doing until the day I die. It's a bit more than we asked of him, I think."

Cable, the MP for Twickenham, said there was now "every prospect" that more MPs from other parties will join the Liberal Democrats and told Business Insider he was "optimistic that they will end up supporting us."

He said his party was "not in a hurry" to recruit more MPs after Umunna joined last week.

"They all have individual agendas and we have issues with our local parties which we need smoothing out," he said.

However, he said that he was in talks with the other five MPs who like Umunna quit Change UK last month. Heidi Allen and Sarah Wollaston are the closest to joining, sources familiar with talks have told Business Insider.

"With the Independent Group, who are not continuing to compete with us, there's every prospect that they will come across... The important thing is that it's comfortable on both sides. I'm optimistic that they will end up supporting us."

Cable said he did not regret stepping down as leader just as the Lib Dems were gaining new momentum.

"It's better to do it in this planned way than a sudden scramble and it means they [Swinson or Davey] are taking over with the wind behind them," he told Business Insider.

Reflecting on his time as leader, Cable said he did not have a "spectacular one-off thing" that he was most proud of.

His told Business Insider that his biggest achievement was showing "patience at a time when a lot of people rubbished us and said we were damaged goods and all of this nonsense."

He said: "The important thing was to be patient and to do the slow, rebuilding job. It was doing the groundwork.

"I stress that although I'm getting some credit for it, the real strength was the team behind it. The people I single out who get the most credit for where we are are those people who stuck in local government at the bad times."

Cable will stay on as an MP until the next election. Until then, he's "happy to be a foot soldier," and has also started to accumulate material for a book he is writing on politicians who have had an historic influence on economic policy.

"I've started with Hamilton in the US, Peel here, and gone on to modern times. Deng in China, Mrs. Thatcher, Trump. I don't necessarily agree with them. They may be awful — but they've changed the way we do economics."

Got a tip? Email this reporter at apayne@businessinsider.com and send him a direct message on Twitter at @adampayne26.