Clegg: Sturgeon and Farage must not give Tories an alibi to duck out of televised pre-election debates

He was speaking after the SNP, Plaid Cymru and the Greens issued a fresh call to be included in the debates, insisting it is "unjustifiable and undemocratic" to leave them out.

Mr Clegg said: "I think what people want to see is the three main Westminster parties represented there. Others will, of course, make their own claims to do so as well.

"I think it is very important, not least because the Conservative party are clearly so reluctant to hold these TV debates altogether, that Nicola Sturgeon, Nigel Farage, the Greens et al, don't unwittingly give the Conservatives an alibi to back out of those debates.

"They must happen and they must happen in a way that I think the public would welcome."

During a press conference in London, Mr Clegg also took a swipe at the SNP over former first minister Alex Salmond's bid to become an MP in the currently Liberal Democrat constituency of Gordon in Aberdeenshire.

He said: "I was very struck by the astonishing arrogance and cockiness about the way in which the SNP apparently now can hand-pick a whole community of 60,000 voters in Gordon and say 'thou shall give a platform for Alex Salmond's ambitions to strut his stuff on the Westminster stage'.

"They may be riding high in the opinion polls, they really need to be careful not to take people for granted.

"However grand you might think you are, and boy does Alex Salmond think he is grand, people do not like to be taken for granted in Gordon or Croydon, in Edinburgh or Sheffield."

Under plans put forward in a joint statement by the BBC, ITV, Sky News and Channel 4, the Prime Minister would take on not only Labour's Ed Miliband and Liberal Democrat Nick Clegg, but also Ukip leader Nigel Farage.

Ukip has previously indicated that it might press for a second, ''if the political landscape continues to change''.

Mr Cameron has raised questions over the plans and indicated that talks would need to take place to find ''a proposal that everyone can agree to''.

SNP leader Ms Sturgeon said: "Following the referendum in Scotland, the political landscape has changed utterly. The SNP is now the third biggest political party in the UK in terms of membership.

"Last month we sent this message to the BBC, ITV, Sky and Channel 4 - to exclude the SNP, the Greens and Plaid Cymru from general election TV debates would be to wilfully ignore this new political landscape. Put simply, it is just not on."

Plaid Cymru leader Leanne Wood said: "Plaid Cymru and the SNP provide an alternative to Westminster's promise of austerity and cuts to public services. As the only parties, together with the Greens, to reject the cuts consensus, it is unjustifiable and undemocratic to exclude our three parties from proposed leaders' debates during the forthcoming UK elections."

Ms Sturgeon criticised Mr Clegg's comments at a Westminster press conference with Plaid Cymru leader Ms Wood and Green Party leader Natalie Bennett.

Ms Sturgeon dismissed as "pretty lame" Mr Clegg's warning that their objections could block debates altogether.

"There is a democratic principle at stake here," she said. "UK politics has changed, there has been a coalition government for the past five years, we live now in a much more multiparty democracy than has ever been the case in the UK before.

"Clearly in Scotland in the period since the referendum we have seen a surge in the Greens but a considerable surge in the SNP as well. The SNP is now the third biggest party by membership in the entire UK.

"It is potentially the case that we are heading for a hung parliament where our parties may have signifcant influence on what happens and who forms a government.

"We are articulating it as a principle that's important to our parties, and it is. But it is a democratic principle that is very important to the electorate as well.

"If I was a voting member of the public in England I would want to hear what the SNP would do in the scenario of a hung parliament."