UK Independence Party leader Nigel Farage has been invited to take part in a TV debate with David Cameron, Ed Miliband and Nick Clegg ahead of next year's general election.
The BBC, ITV, Sky and Channel 4 have announced plans to hold three debates.
The others would be a head-to-head debate between just the Conservative and Labour leaders, and one with them and the Liberal Democrat leader.
But the SNP, the Green Party, Plaid Cymru all criticised the proposals.
The broadcasters said the proposed formats reflected "changes in the political landscape" since the three prime ministerial debates during the 2010 general election, which featured Conservative Mr Cameron, Labour's Gordon Brown and Lib Dem Mr Clegg.
Prime Minister David Cameron said he had always backed the idea in principle but said the broadcasters' proposal raised "some questions".
Referring to the Green Party's absence, he said he could not "see how you can have one party in it that has an MP in Parliament and not another party" that also has one MP.
The prime minister also asked why all the debates were planned during the election campaign, "rather than spreading them out over a longer period".
He added: "I'm sure clever people can get together and sort this issue out. But they do need to take these sorts of points into account."
He later told ITV's The Agenda programme: "I think the Lib Dems and the Greens are already consulting lawyers about opposing what has been proposed so I am sure this won't be the last word."
Labour leader Ed Miliband welcomed the broadcasters' proposals, saying they were "a positive step forward".
He added: "I hope David Cameron is not going to put up false obstacles to these TV debates happening. Frankly he should be saying these TV debates will happen, they must happen during the campaign, for my part I'm going to make sure they do."
UKIP has its first MP, following the success of Conservative defector Douglas Carswell in the Clacton by-election last week. It has also consistently out-polled the Liberal Democrats in recent months.
'Party of government'
Responding to the broadcasters' offer, Mr Farage said: "The decision is better than it could have been. It does at least recognise the increasing popularity of UKIP. However if the political landscape continues to change we would expect and ask for inclusion in a second debate."
But the Liberal Democrats reiterated their previously stated position "that we would be prepared to sign up to the same 3-3-3 system we had in 2010".
A spokesman added: "We do not accept the proposal that the Liberal Democrats, as a party of government, should be prevented from defending our record in one of the TV debates."
SNP Westminster leader Angus Robertson MP said the proposals would be "utterly unacceptable to any democrat".
He added: "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."
Green Party leader Natalie Bennett said: "With these statements the broadcasters are demonstrating just how out of touch they are with the public mood, and how ridiculously they cling to the idea that the future of politics looks like the past.
"It is clear from votes and polls that the public are fed up with the three business-as-usual parties and are looking around for alternatives."
Plaid Cymru leader Leanne Wood said: "Broadcasters have shown themselves to be out of touch by clinging on to the notion that there is no alternative to the tired Westminster elite.
"The people of Wales deserve nothing less than to hear what all parties have to offer them and we will be taking immediate steps to ensure that this happens."
Katie Ghose, chief executive of the Electoral Reform Society, welcomed proposals for debates but said it was hard to justify excluding smaller parties.
The suggested schedule is for debates on 2, 16 and 30 April, ahead of the election on 7 May.
There would be:
- One head-to-head debate between the "two leaders who could become prime minister", Mr Cameron and Labour's Ed Miliband, on Sky and Channel 4 and chaired by Jeremy Paxman
- Another, also to include the Liberal Democrat leader, to be hosted by David Dimbleby, on BBC TV, radio and online
- Another debate, on ITV and chaired by Julie Etchingham, featuring the leaders of the Conservatives, Labour, Liberal Democrats and UK Independence Party
The BBC's chief political adviser, Ric Bailey, said organising debates was incredibly complicated", adding: "We think this is a fair and realistic proposal. What we are really doing now is inviting the party leaders to talk about that."
The suggestions were based on "objective data", including "electoral support, what happened in the past" and "current support, in terms of opinion polls".
Jonathan Levy, of Sky News, told BBC Radio 4's The World at One: " In no way is any part of the political landscape being ignored here."
UKIP had attracted "considerable electoral support in a national election this year", he said, referring to the European Parliament elections in which it topped the polls.
The last general election campaign saw the first televised leaders' debates, attracting 22 million viewers and dominating coverage.
The broadcasters said next year's versions would take into account "developments in digital media" and each is understood to be available to other broadcasters to air.
There are plans to work with social media organisations such as Twitter and Facebook "to ensure the widest possible audience engagement".
The directors of BBC Scotland, Wales and Northern Ireland are writing to the leaders of the main political parties in each nation inviting them to discuss setting up general election debates.
They are proposing debates during the campaign which would be screened in each nation and be available across the UK.
In May, YouTube, the Daily Telegraph and the Guardian announced they were making a joint bid to host their own leaders' debate.
Mr Clegg and Mr Farage also went head-to head in two debates in March and April this year on whether the UK should remain in the European Union.