The Tories have made an official complaint over a BBC programme they say encouraged heckling at a campaign meeting addressed by Michael Howard.
They claim the BBC equipped three hecklers at the meeting in Horwich near Bolton last Wednesday with microphones to "generate a false news story...and embarrass or ridicule the leader of the Conservative Party".
The BBC deny the documentary The History of Heckling is biased but promised a full investigation. In a statement the BBC said the documentary makers had attended meetings held by other parties but a spokesman could give no further details.
The Sunday Telegraph reported that the two men and a woman shouted slogans including "Michael Howard is a liar", "You can't trust the Tories" and "You can only trust Tony Blair".
Conservative head of communications Guy Black wrote a stronglyworded letter to BBC director of news Helen Boaden.
He accused the corporation of breaking producer guidelines that they must remain impartial during election campaigns and therefore breaching the BBC's charter agreement.
The BBC said the documentary, which is currently being edited for BBC3, is "a completely legitimate programme about the history and art of political heckling".
" The programme observes hecklers at other parties' campaign meetings and not just the Conservatives," it said.
The row is the latest in a series of disputes between the BBC and the Conservatives, who accuse the corporation of systematic bias against the right.