The prospect of TV election debates between the party leaders has suffered a huge blow after David Cameron said he will not participate in them if the Green party is excluded.

Nick Clegg immediately responded with a tweet saying: “Come on David Cameron, the broadcasters have invited us, the public expect it, just say yes and stop making excuses.”

The prime minister was responding to a draft ruling by Ofcom, the broadcast regulator, that the Green party does not have sufficient support to qualify for “major party status” in the general election, but Ukip may have. Ofcom’s initial decision makes the Greens’ argument to be included in the TV leader election debates much harder to press successfully, although the party has until early February to make its case before a final decision in early March.

Cameron has been eager to reduce the prominence of the TV debates, partly because he believes they sucked the life out of the rest of the campaign in 2010 and partly because he fears he might not benefit from a head-to-head with Ed Miliband.

@David_Cameron Come on David Cameron, the broadcasters have invited us, the public expect it, just say yes and stop making excuses. — Nick Clegg (@nick_clegg) January 8, 2015

Cameron was careful not to rule out a TV debate completely if the Greens are excluded, but at the very least he has set a difficult challenge for the broadcasters. ITN asked: “Are you saying you are not going to go in as it stands, unless at least Greens are in?” Cameron replied: “Correct. I don’t think the current proposals work. You can’t have one minor party without having another minor party and I think that’s only fair.”

Miliband responded to Cameron’s remarks by saying: “The prime minister should be able to stand on his record and stop running scared.” A Labour official added: “David Cameron is going to fool no one if he wants the public to believe he is walking away from the TV debates on a point of principle about the inclusion of the Greens.”

Natalie Bennett, the Green party leader, said the Ofcom decision was deeply disappointing, stuck in the past and failed to grasp the way politics was changing fast. It ignored the views of young voters, she added.

The Ukip leader, Nigel Farage, branded Cameron “a chicken running scared”.

Paddy Ashdown, the Liberal Democrat campaign manager, accused Cameron of using the Greens to duck TV debates, adding: “Not since the photos of Cameron driving huskies have green issues been so cynically harnessed to Tory interest.”

The broadcasters have proposed three one-hour TV debates, the first involving the Ukip, Liberal Democrat, Labour and Conservative leaders, the second Lib Dem, Labour and Tory. The final debate would be between Miliband and Cameron on the basis that one or other of them is likely to be prime minister.

The Tories have been canvassing opinion to see if they could get away withjust one debate and will have to judge how much long-term damage they would suffer if they were seen as the party that blocked the debates.

Ofcom stressed that it is for the broadcasters to decide who can participate in the TV leader debates, but its provisional ruling will encourage channels to stick to their position that the Greens should be excluded.

Natalie Bennett said the Ofcom decision ignored young voters. Photograph: Mark Kerrison/Mark Kerrison/Demotix/Corbis

Also on the table is a groundbreaking digital debate proposed by a consortium of the Guardian, the Telegraph and YouTube. A spokesman for the three companies said: “The Ofcom ruling does not affect the proposal for a digital leaders debate which would engage an entirely new audience in the national political conversation.”

The parties have failed to reach agreement in principle on the conduct of the debates and likely contestants. Neither the Liberal Democrats nor Labour want to discuss the negotiations, fearing it would give the Tories an excuse to pull out.

Ofcom, in its consultation issued on Thursday, said the decision on major party status would affect not only party election broadcasts but also the total amount of editorial coverage the parties receive for the election. It would mean, for instance, that commercial broadcasters would try to ensure that Ukip had more coverage than the Greens, but not necessarily as much as the other three parties.

The Tory MP Zac Goldsmith, a prominent environmentalist, was among critics and said it was a “disgraceful, indefensible decision by Ofcom”.

In its statement, Ofcom said its initial view was that the Green party (including the Scottish Greens) had not “secured sufficient support in previous elections and current opinion polls to be added to Ofcom’s major party list for the purposes of the May 2015 elections”.

It went on: “The Green party has not demonstrated significant past electoral support in general elections. The Green party has performed better in some other forms of election, such as the 2014 European parliamentary elections, obtaining 8.0% and 8.1% of the vote in England and Scotland.

“In terms of evidence of current support, the party’s opinion poll rating in Great Britain-wide polls has increased in recent months to 5.9% in December 2014 (4.0% on average during 2014).”

Of Nigel Farage’s party, Ofcom stated: “Ukip has not demonstrated significant past electoral support in previous general elections, achieving 3.5% of the vote in 2010 and has not won a parliamentary seat at a general election.

“Ukip’s performance in a number of other significant forms of election has, however, been stronger. Notably, Ukip has won two seats in parliament at recent byelections.

Nigel Farage labelled the prime minister a chicken. Photograph: SEAN SMITH/sean smith

“Ukip demonstrated a significant level of support in England and Wales in the European parliament elections in 2014 (29.2% England, 27.6% Wales) while its share of the vote was lower in Scotland (10.5%). Ukip also demonstrated significant levels of support in the English local elections (15.7% in 2014 and 19.9% in 2013).

“Opinion poll data indicates that Ukip currently has significant levels of support in England and Wales and has the third highest rating in those polls after the Conservative and Labour parties. Opinion poll data in Scotland shows lower levels of current support.”

The list of major parties is important for Ofcom’s regulation of election coverage, in particular requiring the relevant broadcasters to allocate at least two party election broadcasts to each major party ahead of an election.

Ofcom said its current list of major Great Britain-wide parties consists of the Conservative party, Labour and the Liberal Democrats.

In Scotland and Wales respectively, the major parties are joined by the Scottish National party and Plaid Cyrmu. In Northern Ireland, they are joined by the Alliance party; the Democratic Unionist party; Sinn Féin; the Social Democratic and Labour party; and the Ulster Unionist party.