Northern Ireland’s two biggest parties, the Democratic Unionists and Sinn Féin, are separately considering legal action over broadcasters’ refusal to allow them to take part in two televised election debates.

The DUP said it would consider legal action if the BBC, ITV, Channel 4 and Sky did not change their minds, and Sinn Féin said it was consulting its lawyers.

The DUP has eight MPs at Westminster, and Sinn Féin five, although the republican party boycotts the House of Commons.

David Cameron said this week that it was only fair that Northern Ireland parties be given the opportunity to take part in the debates along with the SNP and Plaid Cymru.

Nigel Dodds, the DUP leader in the Commons and MP for North Belfast, said: “As the fourth largest party in the House of Commons, the DUP has more seats than some of those who are now included in the broadcasters’ proposed format. In a hung parliament scenario, the DUP’s votes will be crucial after the election, therefore it is right that we play a role in the debates.”

The nationalist SDLP, which has three seats at Westminster, demanded it too be included, but did not state whether it was considering legal action. The party leader and South Belfast MP Alasdair McDonnell said: “If these debates are to be inclusive they must be fully inclusive.”