Tim Farron will use his first conference speech since stepping down as leader of the Liberal Democrats to accuse the Conservatives of 'breaking Britain'.
An evangelical Christian, Farron quit his role after the general election, saying he was 'torn between living as a faithful Christian and serving as a political leader'.
But despite allegations he was forced out, Farron is to insist 'this is the party that lets you think for yourself'.
Strongly opposed to Brexit, he will defend his record and his opposition to leaving the EU even in the wake of the Leave vote.
'We are right, we will be proven right, we must not give up,' he will tell party delegates at their conference in Bournemouth on Monday.
Attacking both the Foreign Secretary, Boris Johnson, and Labour leader Jeremy Corbyn, he will say: 'Patriots are never populists. Because patriots tell their country the truth, it is a treacherous act to tell lies to your country, Boris... or to be a coward, acquiescing while lies gain a foothold, Jeremy.
'So we must tell the truth. Britain's exit from the European Union will make, is making, my country poorer, my country less safe, my country less powerful... and it is damaging the future for our children.'
He said Brexit will reduce immigration but only because it will turn the UK 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!'
His speech comes after a tense session on Sunday where party activists attempted to reverse the policy of holding a second referendum on Brexit in favour of a policy to oppose it at all costs without a second poll.
After a vote on the conference floor, party members backed Farron's flagship policy of putting the UK's final referendum decision to voters in a second referendum.
'I said during the campaign that my motivation for fighting the madness of Brexit was that I wanted to look my children straight in the eye in the years to come and say that I did everything, everything to prevent this disaster,' Farron will say on Monday.
'And that is still my motivation. It is not too late. The Britain we love can still be saved. Do not give up.'
