The UK will become an "impoverished backwater" if it leaves the European Union, Liberal Democrat leader Tim Farron has claimed as the party launches its campaign to retain ties with Brussels.
The Lib Dem leader and his predecessor Nick Clegg are using the party's conference to make the case for voting to remain in the EU in the referendum on membership promised by David Cameron.
Mr Farron warned that quitting the EU would "trash our economy" and leave Britain isolated and lacking influence.
Setting out the economic risks of Brexit, Mr Farron told BBC London: "Of the top 250 companies on planet Earth, 100 of them have as their global or EU headquarters the City of London. Half of them give as their primary reason for being in London that we are in the European Union.
"So if we want to trash our economy, create massive unemployment and hugely reduce Britain's influence and relevance in the world then we leave the EU.
"So there's nothing esoteric about whether we are or are not in the EU, it is absolutely critical to whether Britain succeeds as a country or whether we become an impoverished backwater.
"My great concern is you have got David Cameron facing both ways on this, now sadly Jeremy Corbyn following suit. There is only one party that is going to stand up for Britain's national interests and fight very clearly, without any hesitation whatsoever, and that is the Liberal Democrats."
The party leader, who has claimed his door is "wide open" to Labour MPs and members distressed about the direction their party has taken under Mr Corbyn, said he would be prepared to work with the left-winger despite their differences.
He told ITV's Good Morning Britain: "I'm a pluralist, I'm not somebody who thinks I'm always 100% right, nor that everybody else is 100% wrong.
"Jeremy Corbyn, I think, believes in electoral reform, Britain desperately needs it so I would work with anybody who supports us on those things.
"I'm hopeful that we would find amongst the Labour Party some people who take the kind of humanitarian lead we have on the refugee crisis, I hope that would be the case.
"I know there are people who think it would be madness for the UK to leave the European Union. I'm not sure what Jeremy Corbyn thinks, but I'm sure many of his shadow cabinet agree with me that Britain must stay in the European Union.
"We will be working with people like that, of course."
The Lib Dem conference in Bournemouth will see a return to the political frontline by former deputy prime minister Mr Clegg, who quit following the electoral mauling in May which saw the party reduced to a rump of just eight MPs.
He will use a speech to warn that the UK could be torn apart if the country voted to leave the EU.
Mr Clegg will say: "The stakes could not be higher: not just one, but two, unions now hang in the balance.
"If we vote to leave the EU, I have no doubt that the SNP will gleefully grab the opportunity to persuade the people of Scotland to leave the UK as well."