Labour and Tory moderates can find a new political home by defecting to the Liberal Democrats to avert a chasm opening in the centre ground of British politics, the party’s leader, Tim Farron, has said.

On the opening day of the Lib Dem conference in Brighton, Farron said he was making a simple offer to those in the two largest parties despondent at the post-Brexit direction of Theresa May and Jeremy Corbyn.

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“Across the range of British politics we now see populists of the far left and the far right getting hold of their parties,” he said. “There are people who are liberals in the Labour party and people in the Conservative party who will be feeling increasingly uncomfortable about the direction of their party. My simple offer to those liberals in other parties is, do you know what, maybe it’s time to join a liberal party.”

At a members’ rally on Saturday night, Farron is set to repeat his pledge that the Lib Dems will push for a second referendum on any Brexit deal.He will say he felt like he had lost part of his identity after the Brexit vote and that he would not sit back and accept the result of the June poll.

“There are many people who would argue that being a good loser means accepting defeat and giving up,” he will say. “Too many politicians are only too happy to chop and change their principles, and just go whichever way the wind is blowing. On the 23 June I believed passionately that Britain is better off in the European Union. I still believe passionately that Britain is better off in the European Union.”

Farron will say he believes defeat over EU membership is only temporary. “I will not be told that I am not a European. I will not accept that my country has been lost to the nationalists,” he will say. “Britain can still stand tall in Europe and Britain can still be freed from the Tories. Let’s take back control.”

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The Brexit vote has galvanised several key figures, including the former London mayoral candidate Siobhan Benita and former Royal College of GPs council chair Claire Gerada, to join the party in what senior figures hope are green shoots of recovery after its collapse at the ballot box in 2015 after serving in the coalition government.

Benita, who ran as an independent mayoral candidate in London against Ken Livingstone and Boris Johnson, and Gerada, previously an outspoken Labour supporter, both said they had joined the party in the wake of the EU referendum.

Benita, formerly a top Department of Health civil servant, said she had suggested to the Lib Dem candidate Brian Paddick that they join forces in the 2012 mayoral elections, but had been suspicious of joining any political party herself because of her civil service background. “Brexit was the final ‘stop thinking and actually do it’ moment. I joined the Lib Dems on the day the result was announced,” she said.

Gerada said she had become disillusioned with the Labour party since the last general election, but had been impressed with the Lib Dems in coalition. “I think they did a superb job, but we never really realised that at the time,” she said. “My decision to join the party had been brewing for a while, but the Brexit result was the final straw.”

Ivan Masow, an entrepreneur who unsuccessfully attempted to become the Tory London mayoral candidate, said he had also left his party for the Lib Dems. Massow, a former flatmate of Michael Gove’s, had previously defected to the Labour party in 2000, but later claimed it had been a publicity stunt to focus the Tories on repealing section 28. He rejoined the Conservatives shortly after.

The Lib Dem health spokesman, Norman Lamb, will make the first key speech of conference on Saturday afternoon, and will float the idea of introducing a dedicated NHS tax in order to boost the coffers of the health service.

Other debates on policy motions will include liberalising prostitution laws and the downgrading of the UK’s nuclear capabilities.