The Liberal Democrats have overwhelmingly approved the party’s plan of going into an election with the promise to revoke Brexit without a referendum, despite warnings from delegates at their annual conference in Bournemouth that it risked alienating some voters.

Under the plan, the Lib Dems would still support a second Brexit referendum with an option of remain, but going into an election would promise to revoke article 50 if the party won an absolute majority – which is seen as highly unlikely.

The move comes after the party gained another new MP as the former Conservative universities minister Sam Gyimah, one of the 21 Tories ejected from the party for rebelling in a Commons vote, announced he was joining the Lib Dems.

Speaking earlier, the party’s leader, Jo Swinson, said the policy would allow the Lib Dems to be “straightforward with people in an election”, and that it would be odd for a Lib Dem government to be obliged to negotiate a Brexit deal it did not support just to put it to a referendum.

Among a number of speakers at the conference who backed the policy, saying it would give the party a distinctive edge in an election, was Chuka Umunna, the former Labour MP-turned-Lib Dem, who told delegates it would reinforce the party’s “unequivocal” message on Brexit.

But Niall Hodson, a Lib Dem councillor in Sunderland, which voted heavily for Brexit, warned the strategy could be seen as absolutist and seemed to be aimed at “piling up more and more remain votes in London” rather than expanding the party into new areas.

Tom Brake, the party’s Brexit spokesman in the Commons who formally proposed the idea, said a clear message was necessary. “We will put an end there and then to the Brexit nightmare that is dragging the country down and tearing us apart,” he said.

The conference is taking place amid a buoyant mood for the Lib Dems, who are celebrating record membership and a series of new MPs. As well as Umunna, his fellow ex-Labour MPs Luciana Berger and Angela Smith have joined in recent weeks, along with the former Tories Phillip Lee and Sarah Wollaston, and the Brecon and Radnorshire byelection winner, Jane Dodds.

In a question-and-answer session with party activists, Swinson was heckled by one member over the arrival of Lee, who as a Tory MP argued immigrants should show they do not have HIV or hepatitis B before being allowed into the UK.

As she discussed Lee’s switch of party a Lib Dem councillor in Chelmsford, Catherine Finnecy, shouted that Swinson was supporting a “Ukip policy” due to Lee’s views.

Asked about the new Brexit policy – first revealed in the Guardian – Swinson said there had to be clarity over the issue.

She told BBC One’s Andrew Marr Show: “As Liberal Democrats we have argued that a specific Brexit deal should be put to a people’s vote to give clarity. But if we end up at a general election then I think we need to be straightforward with people and give them an option for all this Brexit chaos to stop.

“But if instead we find ourselves in an election it would be bizarre for us to suggest that the main issue of that election will be anything other than Brexit.”

Swinson, who has ruled out working with either Jeremy Corbyn or Boris Johnson after an election, said: “I think people want politicians to be straightforward with where they stand. I recognise that not everyone agrees with the Lib Dems on this.

“But I think they recognise also that we are being straightforward about our position. It’s what we genuinely think is right for the country, and people should have that chance to choose that outcome.”

Speaking earlier on Sky’s Sophy Ridge on Sunday show, Gyimah said the Conservative party had changed.

“The stark reality is that I had to face up to the fact that the Tory party is in a different place,” he said. “If I want to stand up for liberal values, then the Conservative party is no longer the place to do that.”