The Liberal Democrat leader will accuse Corbyn campaigners of being 'in the pub' rather than canvassing in a Sheffield suburb.
Lib Dem leader Tim Farron will celebrate the party's unexpected win over Labour in the Mosborough by-election last week by accusing Corbyn supporters of being in the boozer when they should have been canvassing.
In his opening speech to Liberal Democrat conference today, he is expected to lay into Momentum, the campaign group backing Jeremy Corbyn's leadership.
Labour suffered a devastating defeat in the Sheffield Council Mosborough ward by-election last week. The Lib Dems came from a distant fourth place in May's local elections to win the seat with a 20 per cent swing.
Just three weeks earlier, around 2,000 people flocked to see Jeremy Corbyn in Sheffield city centre.
Mr Farron will say: "Eight days ago our team in Sheffield kept right on until 10…but they weren’t the only ones working hard. Just yards away from where the Lib Dem team were knocking on doors, some of the local Momentum activists were in the pub.
"They were campaigning. They were telephone canvassing for Jeremy Corbyn in the labour leadership election. Meanwhile in the real world outside that rather nice Sheffield pub, real people were voting in a real election."
Sheffield's branch of Momentum had organised a phone bank session in support of Mr Corbyn's bid to stay on as leader.
The group later said some Corbyn supporting Labour members had been involved in canvassing 'recently.'
Mr Farron will also have a pop at Labour's 'compliance unit' which has been performing a rash of suspensions of party members and supporters for rule breaking ahead of the election.
He will say: "Now, a quick aside, for all you new members here tonight – first of all, welcome and thankyou!
"Welcome and thank you, but just so you know… no one in this party ever gets purged for thinking the wrong thoughts or belonging to the wrong group… no, we are a liberal party.
"But you will get purged for wimping out and knocking off at 9.30pm on polling day. Just saying."
The Liberal Democrats conference kicks off today in Brighton with a supporters' rally at 6.15pm.
Mr Farron will speak again on Tuesday, giving his keynote speech at the conference's close.