Executives from Channel 4 told a House of Lords committee that they were "excluded" from the broadcasts at the last election in 2010, when the BBC, ITV and Sky News each staged a live debate between then Prime Minister Gordon Brown and David Cameron and Nick Clegg. Channel 4 instead televised a debate between would-be chancellors of the Exchequer.
No agreement has yet been reached on whether the debates should be repeated in 2015.
Intense negotiations are under way over whether the number, timing and format for any debates should follow the pattern set in 2010, when three debates were held at weekly intervals, with each leader given fixed times to answer questions in turn.
Liberal Democrat leader Mr Clegg has said he is ready to sign up for debates, but Prime Minister Mr Cameron is yet to commit himself. Labour has said its priority is to secure a televised clash between the two "prospective prime ministers" - Mr Cameron and Ed Miliband.