LONDON (Reuters) - Britain’s Liberal Democrats are to set out on Tuesday their plan to stop Brexit, with party leader Vince Cable appealing to opponents in all parties to put aside “tribal differences” and fight a divorce from the European Union.

Vince Cable, the leader of Britain's Liberal Democrat Party, appears on the BBC's Marr Show in London, September 17, 2017. Jeff Overs/BBC handout via REUTERS

The fourth largest party with 12 representatives in the 650-seat parliament, the Liberal Democrats are hoping to become the lightning rod for any rise in anti-Brexit sentiment as Prime Minister Theresa May’s government edges closer to leaving the EU in March 2019.

Cable, who held one of the most senior roles in the 2010 to 2015 Conservative-Liberal Democrat coalition government, has redoubled his party’s criticism of the decision to leave the EU since becoming party leader in July.

“A disaster looms: Brexit,” Cable will say in his address to the party’s autumn conference.

“In the real world, we have yet to experience the full impact of leaving Europe. But we have a taste of what is to come in the fall of the value of the pound,” advance extracts of Cable’s speech said.

A former business minister, Cable will urge a new referendum be held and argue that a fall in sterling shows the damage leaving the EU will do.

Sterling fell by as much as 20 percent against the dollar in the months following last June’s EU referendum. It has recovered around half those losses, in part thanks to increasing expectations of an interest rate hike to ease inflation, but the pound is still 9 percent down on pre-Brexit levels.

“Foreign exchange dealers are not point scoring politicians. Their cold, hard, unsentimental judgment has been, quite simply, that Brexit Britain will be poorer and weaker after Brexit than if we had decided to stay in Europe,” Cable will say.

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Opposition to Brexit - Britain’s most far-reaching policy decision in decades - has been emboldened after May failed to win a clear mandate for her exit strategy at a snap election in June. She lost her majority in parliament but kept power thanks to a deal with a small Northern Irish party.

Cable will say Britons have a democratic right to a second referendum once the final terms of the Brexit deal become clear.

“We believe the public have a right to change their mind,” he will say.

Cable, whose centrist party went from junior coalition partners to near extinction in a 2015 election, made only small political gains in the June election having set their stall out as the most pro-EU party in British politics.

But, Cable has previously said he expects dissatisfaction with Brexit to grow as the negotiations develop, and will on Tuesday call for opponents from all parties to come together to oppose the divorce.

“We have to put aside tribal differences and work alongside like-minded people to keep the Single Market and Customs Union, essential for trade and jobs,” he will say.