Jeremy Corbyn and Owen Smith are not holding anything back with less than a fortnight before the winner of the latest Labour leadership election is announced on 24 September. Smith accused Corbyn of being "delusional" over the party's electoral prospects, while the Labour leader's campaign said the challenger was "resorting to insults".
"With Jeremy we've got no chance of winning," the former shadow work and pensions secretary told the BBC.
He added: "If there was an election tomorrow Labour would be decimated, and that's got to be a shock to Labour's system."
Corbyn's campaign hit back at Smith, claiming that the "wheels are coming off his campaign".
A Jeremy for Labour spokesperson said: "Rather than repeatedly talking down our party, and refusing to accept the outcome of this contest, Owen Smith should reflect on his use of such a divisive approach for the rest of this contest.
"Labour members won't appreciate his campaign's 'Project Fear' or 'Project Smear' tactics.
"In contrast, Jeremy has made clear his commitment to oppose the policies of this Tory government which are harming working families in our country, and put forward his positive alternative so that we can get Labour back into government to rebuild and transform Britain so that no one and no community is left behind."
The heated exchange comes ahead of the latest Labour leadership TV hustings on Sky News at 20:00 BST tonight (September 14). Corbyn, who secured almost 60% of the vote in last year's leadership election, is expected to retain the top job.
The bookmakers have the left-winger as favourite and the latest opinion poll from YouGov for The Times of more than 1,200 Labour members between 25 and 29 August, put Corbyn 24 points ahead of Smith (62% versus 38%).