BBC, ITV, Channel 4 and Sky News plan to include Ukip leader Nigel Farage in election debates but not Greens and SNP

As the row over the format of the televised leaders’ debates escalates, the SNP and the Green party have said that they will consider legal action following their exclusion from initial plans announced by broadcasters.

The Green party is considering legal avenues with Caroline Lucas, the Green party’s Westminster MP, tweeting on Monday afternoon that she was “taking legal advice over Green exclusion from TV debates”.

The Guardian understands the SNP does not rule out legal action up to judicial review, but will begin by making direct contact with the broadcasters.

The BBC, ITV, Channel 4 and Sky News have jointly agreed to a plan for three TV debates and written to the leaders of the Conservatives, Labour, Liberal Democrats and Ukip on Monday morning with their proposal, which amounts to an opening of negotiations with the various political leaders.

Their plan is for three debates: Channel 4 and Sky News would co-produce a debate with prime minister and Tory leader David Cameron and his Labour counterpart Ed Miliband hosted by Jeremy Paxman; the BBC would host another with Cameron, Miliband and Lib Dem leader Nick Clegg with David Dimbleby in the chair; and all three would face Ukip leader Farage in the third debate, produced by ITV and chaired by Julie Etchingham.

An SNP spokesperson said: “We will consider our options a step at a time – at this stage we want to discuss these unacceptable proposals with the broadcasters.”

In 2010, the SNP won permission for a judicial review of the BBC’s refusal to give the party the same status as Labour, the Conservatives and the Lib Dems in the series of general election leaders’ debates but withdrew the action in June of that year because the election period had passed.

Broadcasters argued that the then SNP leader Alex Salmond should not take part because the debates were “prime ministerial”, a criterion which the party believes no longer stands with the inclusion of Farage.

Following a surge in membership after Scotland voted no to independence in last month’s referendum, the SNP is now the third-largest party in the UK, having overtaken the Lib Dems.

The SNP’s Westminster leader Angus Robertson MP earlier described the proposals as “utterly unacceptable”, saying: “The broadcasters have the cheek to say that their proposed format factors in ‘changes in the political landscape’ to justify including UKIP – entirely ignoring the fact that the SNP are now by far the third-largest political party in the UK.”

“Current Westminster voting intentions put the SNP in the lead in Scotland, and it is clearly wrong that the leader of the third biggest political party in the UK should be shut out of these network debates.”

Responding to the news that BBC Scotland intends to exclude the Scottish Green party from its general election debate, the party’s co-convenor Patrick Harvie said the broadcast would “fail to reflect the truly unique situation Scotland now finds itself in [following the referendum]”.

• To contact the MediaGuardian news desk email media@theguardian.com or phone 020 3353 3857. For all other inquiries please call the main Guardian switchboard on 020 3353 2000. If you are writing a comment for publication, please mark clearly “for publication”.

• To get the latest media news to your desktop or mobile, follow MediaGuardian on Twitter and Facebook.