Political parties from Northern Ireland should be included in the general election television debates, David Cameron has said, as he rowed back from suggestions by the Tory chairman Grant Shapps that Downing Street is happy with the new format.

The prime minister welcomed a proposal tabled by the main broadcasters last week that will give the Green party, Plaid Cymru and the Scottish National party places in two seven-strong debates. But to cheers from a Tory election audience in Eastleigh, Cameron questioned why Northern Ireland parties were missing out.

Cameron said good progress was being made in the debate negotiations after the broadcasters accepted his recommendation that the Greens should be given a place. “I was told that it was appalling and outrageous that I had suggested that you couldn’t have one minor party without having the other minor party,” he said. “I am delighted the broadcasters have gone away and thought again they have actually come up with rather more minor parties than I had in mind.

“But I am sure they have thought it all through and they know what they are doing, although I don’t quite see why Northern Ireland seems to be missing out. As far as I am concerned that is as important a part of our United Kingdom as Wales or Scotland. But we are making good progress and I am sure they [the broadcasters] know what they are doing.”

The prime minister declined to give a definitive commitment to take part under the current proposed format – two debates with seven party leaders and one debate between himself and Ed Miliband.

“I want to take part,” he said. “I said they needed to do the minor party thing and they’ve certainly done that. They [including the minor parties] were never my terms. I simply said you have a problem if you have one minor party, Ukip, and you don’t have another minor party, the Greens. I want these debates to go ahead and that is good progress.”

Cameron did not demand the inclusion of Northern Ireland’s two main parties, the Democratic Unionist party and Sinn Féin. But his remarks, which contrasted with Shapps’ upbeat response to the revised proposals, suggest Downing Street may be looking at new ways of delaying agreement. Tory sources have made clear in private that the prime minister would like either to kill off the debates or ensure that so many parties take part that they lose their impact and deprive the Ukip leader, Nigel Farage, of a high-profile platform.

Shapps said on Sunday that politicians and broadcasters were “edging towards a solution”. Asked whether he was saying a yes or a no to the prime minister’s involvement, he replied: “It’s a yes.”