Jeremy Corbyn has already "thrown in the towel" over the EU, Liberal Democrat leader Tim Farron will claim.
In a bid to win over Labour voters, Mr Farron will say the embattled leader has let down his pro-European supporters.
Mr Farron who has pledged to campaign for a second referendum will insist that the Liberal Democrats are the only party prepared to fight for Britain and its place in single market.
On the third day of the party's conference in Brighton, Mr Farron is expected to say: "He didn't make an effort to keep us in Europe during the referendum campaign and now he has made clear he won't fight for our membership of the single market.
"Under Jeremy Corbyn, Labour is simply not doing its job as the opposition - and nowhere is that clearer than on Europe. So if he won't, we will.
"My message to pro-European Labour supporters is clear: Jeremy Corbyn is not on your side.
"The Liberal Democrats are the only party fighting to keep Britain in the single market. We are the real voice of opposition to the Conservative Brexit Government and the only party fighting to keep Britain open, tolerant and united."
Mr Farron wants the eventual Brexit deal negotiated by the Government to be put forward to the public in a referendum, with the option of remaining in the EU. On Monday, party members are set to endorse the proposal.
Mr Farron has made clear he believes that no deal could be better than staying in the bloc and so would campaign to overturn the Brexit vote.
But former business secretary Sir Vince Cable has warned that a second referendum "is not a panacea to anything" and urged the party to accept the June 23 result and consider what concessions it is prepared to make.