Former Liberal Democrat leader Tim Farron is to accuse the Conservatives of “breaking Britain” in a bid to cut immigration.
At the party conference, he will say Brexit will turn the UK into a “poorer, meaner, insular place…no-one in their right mind will choose to come here”.
Leaving the EU will reduce immigration without changing any laws, he will say.
It is Mr Farron’s first Lib Dem conference speech since stepping down as leader in June.
He said he was “torn between living as a faithful Christian and serving as a political leader”.
Sir Vince Cable replaced him just over a month later after no-one stood against him.
Speaking to the party in Bournemouth, Mr Farron will say: “There is one promise that Brexit will fulfil. It will reduce immigration without changing a single law.
“Because if you turn Britain into a poorer, meaner, insular place, no-one in their right mind will choose to come here.
“So the Tories are breaking Britain to repel the immigrants. And they do it with Labour’s shameful connivance. What a disgrace.”
‘Courage, not cowardice’
Mr Farron supported the Remain campaign in the referendum and has called for a second vote on the terms of any Brexit deal.
He had said the UK did not have “any chance” of getting a better deal from Brexit talks than it had being a full member of the EU.
The four-day conference is set to focus on Brexit with a debate at the end of the week on the UK’s future relationship with the EU 27 (the other remaining countries in the European Union).
He will also say: “You can be a Corbyn or a May and change your mind on Europe to suit the weather, too afraid of the people to ever deserve to lead them.
“Leadership requires courage, not cowardice. We stand between two parties led by cowards and leading Britain to disaster.