Green co-convener Patrick Harvie is furious that his party has not been included in the live debate involving Nicola Sturgeon, Jim Murphy, Ruth Davidson and Willie Rennie.
The four leaders of the largest parties at Holyrood have signed up to the event, which will be broadcast live from the Assembly Rooms in Edinburgh on Tuesday 7 April between 8pm and 10pm.
The debate will be chaired by STV’s political editor Bernard Ponsonby and a special edition of STV’s political programme Scotland Tonight will be shown at 10:30pm featuring highlights and analysis.
The debate has been billed as one of Scotland’s highlights of the campaign in the run-up to the 7 May general election.
Last night Mr Harvie argued that his party now has more members in Scotland than the Liberal Democrats and had taken part in STV’s referendum debate.
Mr Harvie said: “During the referendum campaign the Scottish Greens played a prominent role, appearing regularly in televised debates and discussions, perhaps most notably at the STV debate at the Assembly Rooms.
“In the context of a post-referendum election, with a politically engaged electorate and a decline in support for two or three parties, it is clear that a broadcaster not reflecting a broader range of voices will not meet the expectations of its audience.”
The Greens have seen party membership increase to more than 8,500 after the referendum, giving them a bigger membership than the Scottish Liberal Democrats, whose last published membership was 2,831.
Mr Harvie said: “I know our many members will be disappointed and our wider support will be calling for STV to rethink their unfair approach. We have the option of consulting lawyers about a challenge.”
Last night STV responded to the Greens’ threat with a spokeswoman for the broadcaster saying: “The debate on 7 April will include all the major parties in Scotland as defined by Ofcom.”
STV referred to Ofcom’s official list of major parties. In a British context, the major parties are defined as the Conservatives, Labour and the Lib Dems. In Scotland, the SNP is added to that list.
Similarly, in Wales and Northern Ireland, Plaid Cymru, the Democratic Unionist Party, the Ulster Unionist Party, Sinn Fein, the SDLP and the Alliance are counted as major parties.
But the Greens believe their exclusion is out of step with public opinion and point out that the UK broadcasters have invited them to take part in two of their three proposed debates.
With five MSPs, the Lib Dems are better represented than the Greens at Holyrood. The Greens have just two sitting MSPs – Mr Harvie and Alison Johnstone.
However, two former SNP MSPs, John Finnie and John Wilson, have joined their party, although they sit as independents.
Gordon Macmillan, STV’s head of news, said: “Live TV debates played a key part in STV’s coverage of last year’s referendum on independence and we anticipate a lively and engaging debate for viewers on 7 April.”
The Greens’ exclusion comes amid controversy over David Cameron’s participation in live TV debates. The Prime Minister has insisted he will take part in only one debate involving seven parties.
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