Vince Cable has warned that Tim Farron would not be a "credible leader" of the Liberal Democrats.
The business secretary suggested that Farron was neither "competent nor reliable" enough to succeed Nick Clegg.
The left-leaning Farron is odds-on favourite to replace Clegg when he stands down. But he sparked anger within the party last week by awarding the leadership "two out of 10" for its handling of the politics of coalition government.
In an interview with BuzzFeed News, Cable said bluntly: "It wasn't at all helpful."
He continued: "I mean, he's a very good campaigning MP, but he's never been in government and has never had to make difficult decisions and I think his credibility isn't great.
"You know, he's an entertaining speaker and has a bit of a fan club. But I suspect he would not be seen as a very credible leader, at least now. Maybe in five, 10 years' time, things are different."
He added pointedly: "The closer we get to an election and the more uncertain it seems, the more people will want people who are seen to be competent and reliable."
Farron, MP for Westmorland and Lonsdale, was also slapped down by former party leader Paddy Ashdown over his comments. Ashdown said he lacked "judgment".
Cable is among those touted as potential rivals to Farron for the party leadership, alongside Ed Davey, Norman Lamb and Jo Swinson.
Asked whether he would stand as leader, Cable said: "We've got one at the moment so the working assumption we're making is that he's going to continue."