Election 2015: NI parties not included in TV election debate plans
New proposals for TV election debates do not include parties from Northern Ireland, it has been confirmed.
The plans by broadcasters feature two debates with seven parties, including UKIP, SNP, Plaid Cymru and the Greens.
Earlier, four parties with Northern Ireland MPs said they would be willing to take part.
DUP leader Peter Robinson said he would write to the BBC and ITV to ask why his party has not been included, in spite of being fourth largest at Westminster.
Sinn Féin, the SDLP and Alliance have all said they should be involved.
The new proposals include plans for two debates featuring the leaders of the Conservatives, Labour Party, Liberal Democrats, UKIP, SNP, Green Party and Plaid Cymru.
David Cameron had previously refused to take part unless the Green Party was included, saying "all national parties" should be represented - prompting a political row.
There are also plans for one debate between David Cameron and Ed Miliband.
The broadcasters said the debates would go ahead regardless of whether any party leader refused to take part.
Roger Mosey, a former editorial director of BBC TV News, said the political debate in Northern Ireland should be kept separate.
"If you put the DUP in, you've then got the SDLP, Sinn Féin, the Alliance, the official unionists and that really is Northern Ireland politics which doesn't translate to the whole of the UK," he said.
"So, I hope there will be measures to make sure that viewers in Northern Ireland will get proper debates featuring Northern Ireland politicians in Northern Ireland, but I think once you start putting Northern Irish politicians into the UK debate it becomes completely unworkable."
Former Northern Ireland Secretary of State Owen Paterson said Mr Cameron's position had now been justified, but said he agreed with Mr Robinson that the DUP should also be invited.
"Well, I think he's been vindicated because we've got the Greens in the debate," he said.
"I think what's bizarre about this decision when the SNP are there with six, and Plaid are there with three, and the Greens have one, I do totally sympathise with Peter Robinson with eight MPs being excluded. My view is the more the merrier."
Sinn Féin said it would "challenge any attempt to discriminate against us".
"Sinn Féin will take every opportunity to present and promote its policies and positions," a party spokesman said.
SDLP leader Alasdair McDonnell said that if one smaller party is included they all should.
Alliance's sole MP Naomi Long said that if the TV debates pool was widened it should be done on a equal basis.
Prime Minister David Cameron has refused to take part in the debates unless the Green Party is included.
On Friday, the DUP's deputy leader Nigel Dodds told BBC's Daily Politics show that it would see what Ofcom and broadcasters have to say and then consider legal action.