Vince Cable has told his MPs he will not remain Liberal Democrat leader in the long term, but hopes to push through radical reform of his party’s rulebook before stepping aside.

Cable is due to make a speech on 7 September, a week before party members gather for their annual conference in Brighton.

He is expected to outline proposals for sweeping changes to his party’s rules, including introducing a new category of registered supporter, similar to the hundreds of thousands who signed up to back Jeremy Corbyn in Labour’s 2015 leadership contest.

Another rule change under consideration is to allow non-MPs to stand for the party’s leadership – a measure that would spark speculation about whether Cable wants to make the Lib Dems a vehicle for realignment of the centre ground in British politics.

Allies denied reports that Cable would use the occasion to trigger a leadership contest in the near term, saying that he would not announce a date. They did acknowledge, however, that while Cable would continue as party leader for an early general election – if Theresa May is forced to call one next year, for example – he would be unlikely to stay on until 2022, when the next general election is due if parliament runs its course.

Other Lib Dem sources suggested there was a widespread assumption, including among MPs, that Cable would step down once Britain had left the EU. Some MPs believe his authority will start to drain away once it becomes clear he plans to step aside.

Despite Labour and the Conservatives being deeply riven over Brexit, the Lib Dems have failed to make advances in the polls under Cable’s leadership. The latest Guardian/ICM poll has the party at 8%, with Labour and the Tories tied on 40%.

Cable was business secretary in the Conservative-Lib Dem coalition government. He lost his Twickenham seat in 2015, but regained it in last year’s general election, and took over as party leader from Tim Farron.

Farron, who remains an MP, stepped aside in the aftermath of last year’s general election after he was repeatedly pressed during the campaign about his personal beliefs on issues including homosexuality. That year the Lib Dems performed below expectations in what many thought would be a Brexit-focused campaign, returning 12 MPs against the eight of two years earlier.

Cable has made keeping Britain in the EU the centrepiece of his policy platform, calling for an “exit from Brexit”. He has also made speeches on other major policy issues, including suggesting a breakup of giant tech companies. Some activists, however, express private concern about his low media profile.

The former energy secretary Ed Davey could stand in a future leadership contest, after declining to do so last year for family reasons. Cable’s well-regarded deputy, Jo Swinson, who is currently on maternity leave, is also widely seen at Westminster as a potential successor.

Daisy Benson, the party’s former prospective parliamentary candidate for Yeovil, highlighted the Lib Dems’ gains in May’s local elections, in which they took councils including Richmond and Kingston in London, and said she welcomed reforms that could broaden the party’s supporter base.

“Our council results are really excellent, but we do need to be a lot more ambitious,” she said.

The package of party reforms under consideration has been partly inspired by Justin Trudeau’s Liberal party in Canada. Trudeau broadened his party’s supporter base as he swept to power.