Exclusive: Sir Vince Cable will give a major speech on the future of the Liberal Democrats next month.

The speech is set to take place on Friday, September 7.

Two well-placed sources suggested Cable will reveal his plan for an early resignation.

BI has also seen WhatsApp messages in which senior Lib Dem figures discuss Cable's rumoured departure.

Sources close to Cable deny knowledge of resignation plans.

The party is considering implementing radical reforms which would let non-MPs stand to be leader and non-members vote in leadership contests.

LONDON — Sir Vince Cable is set to give a major speech on Friday, September 7, multiple sources have told Business Insider, amid speculation that he planning an early resignation as leader of the Liberal Democrats.

Senior sources have told BI that Lib Dem members will be invited to a speech in the first week of September, one week before the party's autumn conference, in which Cable will make announcements about reforms and the party's future.

Two senior sources — a well-placed Lib Dem and a party grandee — suggested that the MP for Twickenham will reveal his plan to make way for a successor before the next scheduled general election in 2022.

The well-placed source claimed his departure could come as early as the party's spring conference in March.

BI has also seen WhatsApp messages sent by a Lib Dem MP, in which they claim they "saw it [Cable's rumoured resignation] coming," and agreed to "keep September 7 free."

"Around 20 people know about it," the party grandee told BI, adding that Lib Dem MPs, peers and other figures have been told to ensure they have no plans on the day of Cable's speech.

The same source claimed that the party had discussed holding a "special conference" in November where the rules of a future contest to replace Cable as Lib Dem leader would be established.

A source close to Cable said they were not aware of any plans for him to announce his resignation timetable next month. Cable told BI this summer that he had no immediate plans to quit as leader of the Lib Dems.

Current favourites to succeed Cable are his deputy Jo Swinson; MP Layla Moran; and former minister, Ed Davey.

Jo Swinson (left) and Sir Vince Cable. REUTERS/Neil Hall

Cable's September speech will come as the anti-Brexit party undergoes a process of transformation.

Members will soon decide whether to adopt a series of radical reforms, including allowing non-MPs to stand in leadership contests and non-party members vote in them.

The proposals – known internally as "Project Ozark" — are designed to make the party more inclusive and reach a wider audience. Tom Pitfield, a political strategist and friend of Justin Trudeau, helped Cable come up with them.

The party also has a new slogan, "Demand Better," which will be unveiled at its autumn conference in Brighton.