The 62-year-old is said to have told a group of panellists and Westminster aides after the recording of Thursday's show that Left-wing supporters were telling producers they were Reform supporters in order to secure a place in the audience – and would then lambast the Reform panellist as soon as the cameras were rolling.
The BBC insisted it did 'not recognise that characterisation'.
Reform MP Suella Braverman was embroiled in fiery exchanges with the audience last week after saying during a debate on migration that she does not consider herself English, despite being born in London.
She instead described herself as British of Asian ethnicity. She was accused of 'reinforcing the racist argument' and 'being racist against herself'.
A source said: 'Fiona said afterwards that audience members were lying to BBC producers by pretending to be Reform supporters when they are not and then, when the show has begun, laying into the Reform panellist.
'She said: "It's not our fault if people lie and say they're Reform supporters to get themselves into the audience."'
A Reform UK source said: 'The producers of Question Time claim to have a politically balanced audience, but when the cameras stop rolling even the programme's host, Fiona Bruce, admits that this isn't true.
Fiona Bruce has admitted that the BBC's Question Time audience is packed with Reform UK critics, it was claimed
'In fact, they operate a slipshod vetting system that encourages anti-Reform audience members to participate in plain sight.
'It's for reasons like this that a Reform UK government will scrap the TV licence fee.'
But a BBC spokesman said: 'The Question Time team vets every audience applicant, acknowledging the risk that someone may apply under false pretences. Everyone who applies is spoken to by a researcher and must provide links to their social media accounts. The team also carry out a series of background checks to ensure we have a representative audience with a diverse range of opinions.
'We do not recognise this characterisation of events at all. Fiona is often asked about the makeup of our audience and is always clear about the robust processes we have in place to ensure people are who they say they are, as she made clear on Thursday’s programme.'