The public broadcaster believed the tape was the "smoking gun" which would back up a controversial BBC report on claims that Prime Minister Tony Blair's office exaggerated the contents of a dossier on Iraq in order to beef up the case for military action, the Guardian newspaper reported on Wednesday.
The tape recording of a conversation between Kelly and Susan Watts, the science editor of the BBC's Newsnight programme, was expected to be submitted in evidence to a judicial inquiry into the scientist's death.
The BBC said that it would not discuss the content of the tape. The state broadcaster is thought to regard it as a useful part of its evidence for the inquiry supporting its controversial story, rather than the centrepiece.
Kelly, a defence ministry consultant, was found dead with his wrist slashed in a wood in Oxfordshire, west of London last week, days after coming under intense scrutiny as the source for the BBC story.
After his death, the BBC confirmed that he had been the main source for three journalists who had reported concerns about the way Iraq weapons intelligence had been presented before the war was launched on 20 March.
Watts had quoted an unnamed source at length - now revealed as Kelly - questioning a headline-grabbing claim in the government's Iraq dossier that Baghdad could launch weapons of mass destruction within 45 minutes.
"It was a statement that was made and it just got out of all proportion," the source said.
"They were desperate for information, they were pushing hard for information which could be released. That was one that popped up and it was seized on and it's unfortunate that it was."
The political crisis triggered by Kelly's death is Blair's biggest since he came to power in 1997, and has seen him forced to fend off calls for his resignation.
The scientist's apparent suicide sent shock waves through political circles in Britain and has prompted the media to ask who was to blame for the way Kelly was "outed" as a suspected mole.