UPDATE:

Tory statement:

“These new proposals are being considered as part of the ongoing discussions about the debates.”

And Labour:

“We will debate anyone the broadcasters choose to invite and we are pleased to see the clear proposal to have three debates all in the general election campaign. The broadcasters have obviously made a very significant move to adopt wholesale the Prime Minister’s proposals and it surely now not possible for him to maintain his opposition to participating in these debates. We relish the opportunity for Ed Miliband to take on David Cameron directly in a head to head debate.”