Nick Clegg holds out hope that the Liberal Democrats could play a role in a progressive cross-party government within a few years as he predicts that chaotic Brexit negotiations will gradually destroy support for Theresa May’s government.

Speaking before the Lib Dem conference, which opened in Brighton on Saturday, the former party leader and deputy prime minister said that, while the landscape might look bleak for Lib Dems, with just eight MPs, they should not think they will necessarily “be out in the wilderness for a generation”.

“Politics is so volatile these days and this government is so rudderless when it comes to Brexit negotiations that Liberal Democrats should not dismiss the possibility that we might once again need to play a role in putting the country first sooner than people think,” he told the Observer.

Clegg’s conference message to the faithful is that if their party, working together with progressive elements of Labour, the SNP, the Greens and even some on the left of the Conservatives, can offer alternative visions to young people and policy ideas on how to address issues such as intergenerational unfairness, the environment, constitutional reform and Britain’s place in the world, then public support will develop for something other than uninterrupted, Brexit-dominated Tory rule.

Brexit, says Clegg, will “paralyse the government over the next few years” as ministers discover they cannot “have their cake and eat it” by negotiating favourable trade arrangements with the EU without bowing to its rules.

The likely result, he says, will be that the Conservatives, urged on by their members and the rightwing press, will force May into a “panic Brexit” – a hard Brexit that will involve the UK turning its back entirely on Europe – before the next election in 2020, which will severely damage the country.

“I think at that point, particularly if investors start taking fright, and it starts dawning on people that the government does not have a road map, I think that then the public appetite for other parties to provide an alternative will grow.

“It does not necessarily need to be a new amalgam party overnight. It could be. If that gridlock were to lead to a real sense of drift and malaise, it could be a government of national unity of some description, where parties of different persuasions say they will act together for a period of time, in order to get the country out of the corner the Tories have got it into.”

Rather than being dead in the water after five years in coalition with the Conservatives and the resulting mauling by voters at last year’s election, Clegg insisted that the Lib Dems were in some ways in better shape than the deeply divided Labour party. Labour, he argued, had yet to accept “that it will never be able to govern on its own again”.

“We need to get bigger and we are getting bigger,” he said. “Even at the nadir of our fortunes [at the last election], 2.5 million voted for us – it is a million more than voted for the SNP. It is just because of our crackers electoral system that they get 56 seats and we get eight. We are winning byelections.”

He added: “Of course there is a future. There is a huge amount of energy in the country, there is a search for progressive, thoughtful, compassionate, internationalist politics. That energy will go somewhere. It doesn’t dissipate. Any country in my view that takes a massive decision about its own future such as that to leave the EU against the explicit stated wishes of those who will inhabit that future – in other words, the young – is taking a huge wrong turn in the road. They did vote in this referendum in very large numbers. It is their future which we have told them they cannot have. That is a powerful source of energy.”

Opening the conference, Clegg’s successor as leader, Tim Farron, defended his Christian beliefs, saying he did not understand why some people found his continued refusal to say whether he believes gay sex is sinful a concern. “I think people look at my liberalism, my desire to support people’s rights to make whatever choices they want, and I kind of also expect in the same way people – maybe it’s a naive expectation – to respect my beliefs as a Christian,” he said.