He says the former host was so cruel to guests that stars are now reluctant to come on the show for fear of being ridiculed.
Team captain Fielding told The Sun that Mel B needed a lot of persuasion to appear in the latest run.
He said: ‘She was terrified, like we were going to really rib her. The problem is, I think Amstell ruined it for everyone. Everyone thinks they'll get ripped to shreds.’
However the Boosh star believes the show is ‘cosier’ since Amstell quit last year, and that Terry Wogan – who guest hosts nest week’s show – was the best they have had.
Amstell famously caused Ordinary Boys singer Preston so storm off the BBC2 show in 2007 by reading out awful extracts from his then wife Chantelle Houghton's autobiography.
But the programme already had a reputation for cruel humour under Mark Lamarr’s 17-series tenure. In series three, Lemmy of Motörhead walked off after being made a laughing stock.
Here are the Preston highlights: