Plans by four UK broadcasters to exclude the Democratic Unionist Party from TV general election debates are "inexplicable" since there is "a real possibility" that Northern Irish MPs could play "a crucial role" in deciding the next Westminster government, First Minister Peter Robinson has warned.

British prime minister David Cameron, who does not want to take part in the debates, insisted last week that the Green Party had to be brought in if UK Independence Party (Ukip) leader Nigel Farage was to be given a place by the TV companies.

Now, however, the TV companies have called Mr Cameron's bluff, proposing two debates: the first hosted by the BBC and ITV should involve Mr Cameron, Labour leader Ed Miliband, and leader of the Liberal Democrats Nick Clegg, along with the Greens, the Scottish National Party, Plaid Cymru from Wales and Ukip.

Meanwhile, the second debate, to be hosted by Channel 4 and Sky, would be a head-to-head between Mr Cameron and Mr Miliband - on the grounds that only they could become prime minister after May.

In a joint statement, the four TV companies upped the ante, saying: “The party leaders have been formally invited to take part in these debates. If any decide not to participate, the debates would take place with those who accepted the invitation.”

However, Mr Robinson wrote to the TV companies, saying the decision to restrict the debates to seven parties is “inexplicable”.

“The broadcasters’ decision cannot logically or legally be defended,” he went on, particularly since the DUP is the fourth largest party in Westminster, ahead of the SNP, Plaid, the Greens and Ukip.

‘Equal status’

Northern Irish MPs are "of equal status to MPs elected in any other part of the United Kingdom", said Mr Robinson: "After this election, there is a real possibility that Northern Ireland MPs could have a crucial role in the formation of the next United Kingdom government.

“A debate involving those who could credibly become prime minister or indeed a party with substantial support and members of parliament across the UK could be defended,” he said.

A separate debate including all the significant regional political parties could be defended: "What is not acceptable is a situation where the largest party in Northern Ireland is to be excluded, while regional parties from Scotland and Wales, with fewer seats in Parliament than the DUP, are to be included," he went on.

Urging the broadcasters to think again “as a matter of urgency”, Mr Robinson implied that the DUP will go to the courts to enforce its right, warning that the DUP “will not accept being excluded” if parties such as Plaid and the SNP are invited.

“We will avail ourselves of all suitable remedies to ensure that (NI) is not treated differently to the other regions and the (DUP) is not treated differently to parties with less support at Westminster,” he said.

Meanwhile, Sinn Féin - which confirmed on Thursday it will not abandon its abstentionist policy in Westminster - said, nevertheless, that it would “challenge any attempts to discriminate against us” and take every opportunity to “present and promote its policies and positions”.

The SDLP - which could form part of the arithmetic of the next Commons majority if Labour falls just short of a majority - demanded a place, too, if smaller parties are to be given a place, a view shared by the Alliance Party.

‘Running scared’

Labour leader Ed Miliband, who has accused Mr Cameron of “running scared”, welcomed the latest proposal from the TV companies: “We will debate anyone the broadcasters choose to invite,” a Labour party spokesperson declared.

The leader of the Green Party in England and Wales, Natalie Bennett, describing the TV debates offer as "ground-breaking", reflecting the sharp rise in membership her party has enjoyed in recent months.

Explaining the reasons for the decision to widen the invitation list, the TV companies said it had not been possible to get agreement on the original offer of the Conservatives, Labour, Liberal Democrats and Ukip.

Since then, they said they had “continued to monitor the electoral landscape, as we promised to do, taking into account the polling evidence, and the expressions of public support for the debates to go ahead and for a wider range of parties to be included in the debates”.