Lib Dem leader Nick Clegg has said the broadcasters should come up with "other proposals" for the TV election debates.

David Cameron has refused to take part in the debates unless the Green Party is included.

Mr Clegg told the BBC he thought the debates would go ahead, but suggested the proposed format needed to change.

Earlier this week Mr Clegg, the deputy prime minister, said the broadcasters should grow a "backbone" and push ahead without Mr Cameron if necessary.

The plans put forward by the BBC, Sky News, ITV and Channel 4 in October would include the Conservatives, Labour, Liberal Democrats and UKIP.

One would feature a head-to-head between Mr Cameron and Labour leader Ed Miliband, another would also include Mr Clegg, and the third would feature the leaders of the Tories, Labour, Lib Dems and UKIP.

The suggested schedule is for debates on 2 April, 16 April and 30 April, ahead of the UK-wide poll on 7 May.

'Alibi'

Mr Cameron has said all "national parties" must be represented at the live televised pre-election debates - which were first introduced for the 2010 general election.

Labour leader Ed Miliband has said it was a "pathetic excuse" to avoid the debates, but Mr Cameron said Mr Miliband was "chickening" out of debating the Greens.

On the BBC's Andrew Marr show on Sunday, Mr Clegg said Labour and the Conservatives were using the Greens as an "alibi" to block the debates.

"They want to put the genie back in the bottle. They want to run things as they've done before. I think it's too late," he said.

Image caption Ms Bennett challenged Mr Clegg to say whether he would lobby for the Greens to be included in the TV debates

Mr Clegg, Mr Miliband and UKIP leader Nigel Farage have written to the prime minister saying it would be "unacceptable" for him not to appear.

In separate but identical letters sent earlier this week they urged the broadcasters to press ahead with the debate before the general election.

Mr Clegg said: "The broadcasters need to come forward with other proposals because clearly the current one - which I'm not wildly happy about because it excludes me as a leader of a governing party - so they need to come forward with a proposal.

"I'll get my soapbox out any day of the week."

Asked by Green Party in England and Wales leader Natalie Bennett - who was also on the programme - whether he would lobby for her party to be included, Mr Clegg said it was not for politicians to tell the broadcasters what to do.

Ms Bennett says the polls and public opinion support the argument for the Greens to be included.