Britain's major broadcasters have rejected Prime Minister David Cameron's call for a televised debate between party leaders to be held before the start of the general election campaign.

Instead, the four broadcasters - the BBC, ITV, Sky and Channel 4 - announced they will stick to their previously-announced plans for three debates during the campaign period, and urged the Prime Minister to "reconsider" his refusal to take part in these shows, including a head-to-head showdown between Mr Cameron and Ed Miliband.

Downing Street wrote to the broadcasters on Wednesday with what Mr Cameron's director of communications Craig Oliver said was the PM's "final offer" of a single 90-minute debate featuring at least seven leaders, to take place ahead of the formal start of the campaign on April 30.

But in a joint statement, the broadcasters said they would stick to plans for a seven-way debate involving Cameron, Labour's Ed Miliband, Nick Clegg of the Lib Dems and the leaders of the Greens, Ukip, Scottish National Party and Plaid Cymru on ITV on April 2, followed by a second show on BBC featuring the same line-up on April 16 and a final one-on-one clash between the Tory and Labour leaders on Sky News and Channel 4 on April 30 - exactly a week before the May 7 election.

In a letter to Mr Oliver, the broadcasters made clear they were ready to go ahead with the debates even if Mr Cameron decides not to take part - effectively "empty-chairing" the Prime Minister.

They wrote: "We very much hope that all invited leaders will participate in the broadcast debates. However, in the end all we can do - as impartial public service broadcasters - is to provide a fair forum for debates to take place. It will always remain the decision of individual leaders whether or not to take part."

Mr Clegg called on the Prime Minister to join the debates in a message on Twitter: "Come on, David Cameron, you haven't got your own way so accept it and take part."

In their statement, the broadcasters said: "Today the BBC, ITV, Channel 4 and Sky confirm they will continue with the series of televised leaders' debates as planned in the general election campaign.

"The broadcasters welcome the fact that the Prime Minister has for the first time agreed to take part in a televised debate.

"However, the group believes there needs to be two seven-way debates of a minimum of two hours each, within the election campaign, allowing time to properly represent the views of all parties, covering a broad range of subjects.

"The broadcasters also believe that a head-to-head debate between the two individuals who could become prime minister - David Cameron and Ed Miliband - is important - something the Prime Minister has publicly supported."

They added: "The broadcasters would like the Prime Minister to reconsider taking part in all of these debates. Twenty-two million people watched the leaders' debates in 2010 and there is a public desire and expectation for them to happen again in 2015

"The broadcasters' proposals have come after extensive work over the last six months to ensure the public have the opportunity to watch televised election debates once more.

"The group have worked in an independent, impartial manner, treating invited parties on an equitable basis. They have listened to the views expressed by all parties and adapted the proposals to take into account electoral support.

"The broadcasters will continue to work closely with all parties invited to take part in the televised debates to bring them to their millions of viewers across the UK. The heads of news of all four broadcasters would welcome the opportunity to meet Mr Cameron, or his representatives, to discuss the debates."

Following stormy exchanges on the debates at Prime Minister's Questions in the Commons on Wednesday, Sky News and Channel 4 said they were ready to move the two-header debate to another date if Mr Cameron and Mr Miliband could agree on a preferred day. The new statement made clear that this offer remains on the table.