The SNP have called on David Cameron to boycott the televised leaders' debates unless they are included - after he threatened a no-show over the Greens.
Mr Cameron issued the warning in a surprise move that appeared to jeopardise the prospects of the highly-anticipated showdowns.
Mr Cameron has been a staunch critic of the decision to leave out the Greens - widely seen as posing more of an electoral challenge to Labour than the Tories.
Asked if he would refuse to take part "unless at least the Greens are in?' Mr Cameron replied: "Correct."
Labour leader Ed Miliband accused the Tory leader of being chicken while Ukip leader Nigel Farage and suggested broadcasters should "empty chair" him.
Angus Robertson, SNP Westminster leader, said that Mr Cameron "should also recognise the democratic deficit in excluding the SNP from television debates.
"The Greens have one MP, and we are extremely sympathetic to their case and definitely agree they should be included, but the SNP already have six times that.
"If David Cameron is arguing for the Greens to be included, he has no case against the SNP."
He added that the SNP's exclusion should be "unacceptable to any democrat".
While the SNP and the Greens have been excluded, Mr Farage has been given a place in the proposal debates.
These would see a head-to head between Mr Cameron and Ed Miliband, a second debate featuring Lib Dem leader Nick Clegg and a third adding Mr Farage to the mix.
STV has also announced plans for a debate in Scotland, but has yet to confirm details.
Earlier the SNP appeared suffer a blow in their campaign for a place when a regulator ruled they were not a "major" UK party.
The SNP denounced the decision as "undemocratic" and said it would challenge the finding.
Media body Ofcom also gave a boost to Ukip, declaring the eurosceptics qualified as a major party in England and Wales.
Ofcom does not get a say on who is included in the TV debates, a decision that is for the broadcasters themselves.
But its ruling will guarantee the anti-EU party more air time south of the border.
Parties judged major 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".
While Ofcom also ruled that the eurosceptics did not qualify as major parties in Scotland and Northern Ireland, Ukip sources suggested they were considering a challenge to that finding.
Some polls put Ukip on similar levels of support as the Lib Dems in Scotland.
Ofcom said its view was based on the recent surge that saw Ukip romp home in the European Parliament elections in England, get its first MEP in Scotland, win two Commons seats and overtake the Lib Dems in UK-wide polls.
The SNP also said it would challenge Ofcom's decision.
Angus Robertson, the SNP's general election campaign director, said the announcement represented a "failure to keep abreast of developments since the referendum."
"The recognition of the SNP as a party of majority government and the UK's third largest political party is key in ensuring fair coverage for the general election - and our exclusion should be unacceptable to any democrat."
Patrick Harvie, the co-convener of the Scottish Greens, said the proposals would lead "further elevate" Ukip in UK-wide TV coverage and "skew" the range of voices Scottish voters hear.
The SNP is already listed as a "major party" in Scotland.
Only three parties, the Conservatives, Labour and the Lib Dems are on the UK-wide "major parties" list, a finding that remains unchanged after the latest decision.
Plaid Cymru are judged to be a major party in Wales while the Alliance Party, Democratic Unionist Party, Sinn Fein, the Social Democratic and Labour Party and the Ulster Unionist Party make up the numbers in Northern Ireland.
The regulator said that the Greens - and Traditional Unionist Voice (TUV) in Northern Ireland - had "not demonstrated significant past electoral support in general elections" to justify inclusion.
Ofcom is currently consulting on the findings.
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