David Cameron has said he will refuse to take part in TV election debates if Green Party leader Natalie Bennett is excluded.
At the moment broadcasters plan to hold three debates. The prime minister, Ed Miliband, Nick Clegg and Ukip leader Nigel Farage have been invited to take part in at least one.
The Greens have been lobbying hard for Bennett be included in one of the debates. However the campaign suffered a serious blow today when Ofcom ruled it was not a "major party" in the same way that the Tories, Labour, the Lib Dems and Ukip are.
Asked this afternoon by ITV News whether he would decline to appear in the debates if the Green Party leader was excluded, Cameron replied: "Correct."
Miliband said the prime minister "should be able to stand on his record and stop running scared".
On Twitter, Farage said Cameron was "a chicken running scared over the TV debates". Ukip MP Douglas Carswell added: "If Cameron refuses to take part in TV debates, with what should broadcasters replace him? Tub of margarine? Bottle champers?"
Lib Dem peer Olly Grender, the party's deputy general election coordinator, said Cameron was "using the Greens as a human shield" to avoid taking part in the debates.
Opposition politicians suspect Cameron wants Bennett included in at least one debate in order to split the centre-left vote in the same way Farage is seen to split the centre-right vote.
In October, a YouGov poll for The Times found that 47% of people backed the Greens being involved in the debates, compared to 32% who opposed their inclusion.
Earlier today Ofcom decided to award Ukip "major party" status. But not the Greens. Parties on the list are guaranteed at least two party election broadcasts (PEBs) on each of the TV and radio channels covered by the system - which does not include the BBC.
Ofcom's Broadcasting Code requires that "due weight must be given to the broadcast coverage of major parties during the election period".
It does not directly affect which party leaders are invited to take part in live TV debates, which is an editorial matter for each broadcaster in direct negotiation with the parties.
Natalie Bennett wants the Green Party in the debates
Bennett reacted with fury to Ofcom's initial judgement. Writing for The Huffington Post UK she said: "Obviously, as leader of the Green Party, I'm deeply disappointed by this decision, but as a voter and citizen I'm also gravely concerned about the possible impact on British democracy if this stance is maintained in the final guidance.
"Ofcom is not only ignoring the views of the 275,000 people who signed the petition calling for the Greens to be included in the broadcasters' proposed leaders' debates, but also the evidence that 79% of the public want to see us in those debates (and 85% of women)."
She went on: "I do think we have people in positions of power who have very fixed ideas about what politics looks and sounds like, think that it's a basically an unchanging wrestle between the traditional big two, with a couple of satellites that squeeze into the gaps without really saying anything different.
"When you put together the support for the Green Parties of England and Wales, and Scotland, and Northern Ireland, with those for the Scottish National Party and Plaid Cymru, there's very strong backing for an anti-austerity alternative to the Coalition-Labour approach, as Nicola Sturgeon, Leanne Wood and I pointed out last month."
This comes after Labour frontbencher Sadiq Khan backed calls for the Greens to be included in the election debates, telling the New Statesman: "What the British public deserve to see is all the leaders, and that includes Natalie Bennett, by the way, having a debate about their vision for the country, their analysis of the last five years, an explanation from Cameron and Clegg how the deficit is still £200bn more than they predicted."
Ofcom's decision has raised eyebrows as the Greens have already started to post better poll ratings than the Lib Dems. Research for Conservative peer Lord Ashcroft put the party on a new high of 8% - up three points - and just ahead of the Lib Dems on 7%.