Tim Farron will warn that “no-one in the right mind” will choose to move to the United Kingdom if Brexit goes ahead.
The former Lib Dem leader will use his first party conference speech since stepping down to accuse Theresa May and Jeremy Corbyn of “cowards” who are “leading Britain to disaster”.
On Sunday, the Lib Dems reaffirmed their commitment to offering voters a referendum on the final Brexit deal struck by the government.
It came as Tory divisions over Brexit erupted once again into public after Boris Johnson was accused of “backseat driving” the Brexit process.
Johnson was also accused of a “clear misuse of official statistics” by theUK Statistics Authority after he used a column in the Daily Telegraph to argue the country would “take back control” of £350m a week in payments to the EU.
In his speech to the party faithful in Bournemouth on Monday, Farron will say Britain’s exit from the EU is making it “poorer, less safe and it is damaging the future of our children”.
“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,” he will say.
“So the Tories are breaking Britain to repel the immigrants. And they do it with Labour’s shameful connivance. What a disgrace.
“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. And people know it. They vote for one because they’re terrified of the other.”
Farron, who led the Lib Dems during the general election campaign, hoped that his overtly pro-EU position would persuade swathes of ‘Remain’ voters to back his party.
However his campaign was widely seen as a disappointment, after the party only increased its number of MPs from 9 to 12.
Farron will also defend his time as leader and will tell party members predictions the party would die under him were wrong.
“The day I took over as leader, one journalist predicted confidently that ‘the party that began with Gladstone will now end with Farron’,” he will say. “I saw those assumptions that we were dead and buried and I resolved that we were going to survive, grow and win again.”