Ed Miliband has upped the pressure on David Cameron to agree to a new round of televised general election leadership debates as he signalled his willingness to see changes to the format.
The Labour leader said the previous format of three debates between the three main party leaders should be a "starting point" but that he was open to moves such as a less formal setting and greater voter participation in any repeat of the 2010 confrontations.
He used a Radio Times article to call for immediate negotiations, accusing his main rival of being the "single biggest obstacle" to them going ahead and suggesting the Tories were keen to deny his cash-strapped party vital publicity.
Britain's first such debates, between Cameron, Gordon Brown and Nick Clegg, were staged on the BBC, ITV and Sky News in 2010 after prolonged negotiations between the parties and the TV companies, which resulted in very strict rules on the style of questioning and the division of time for leaders' answers.
Cameron has since complained that they "took all the life out" of the campaign amid constant speculation over whether the practice would be revived, and if so in what form.
There has been speculation that he could seek to exclude Clegg - now deputy prime minister in the Tory-led coalition - and go head-to-head only with the Labour leader, a move which would be challenged by the Liberal Democrats.
Miliband wrote that he saw the debates as a "crucial part" of his pitch to the electorate but that they were being put at risk by the "same old games" that saw talks over copying the US-style programmes come to nothing for decades until 2010.
"It is a pity that the Conservatives will not even sit down to begin negotiations until later this year - when it will be harder to secure an agreement - and have stalled at every opportunity they have been given to do so," he said.
"I can only assume that Cameron wants his party's deep pockets to be used for maximum advantage and that perceived political self-interest lies behind his party's reluctance to get these debates on.
"But no one should want the outcome of the next election distorted by the number of direct mailshots and billboard posters a party can buy. And, while TV debates will not level the playing field on their own, they can help enable people to make better-informed choices when they cast their votes."
Accepting that there was scope to improve the format, he said he wanted to see "more opportunities for the audience to ask questions and the setting to be less formal, because that would help ensure the real priorities of people are reflected".
"However, because I am not going to give the Conservatives the excuse to walk off the pitch by claiming we have moved the goalposts. The starting point for negotiations should be the agreement Cameron signed up to four years ago: three debates between the three main party leaders over three weeks of the campaign.
"With the election just a year away, it is time Cameron stopped dragging his feet and showed he is willing to debate the future of our country by allowing the negotiations to begin."
Clegg told the Financial Times he would "struggle to think of even half a respectable excuse the Conservatives could come up with to deny the British people the right to see the party leaders measuring up against each other in a leaders' debate".
"Ed Miliband and I said we'll sign up on the dotted line and repeat the format of last time. People found it a useful innovation and I think the Conservatives should not run away from having the kind of debate we had last time."
A recent poll suggested a majority of voters believe Ukip leader Nigel Farage should be allowed to join general election TV debates, given the rising popularity of the eurosceptic party.
Farage has stepped up his claim for an equal billing after emerging the clear winner amongst viewers of two clashes with Mr Clegg ahead of the May 22 European elections.
But Cameron insists the debates "predominantly should be about people who have a prospect of becoming prime minister".