NICOLA Sturgeon has not ruled out the possibility that the SNP could take legal action to try to ensure the party, now the UK's third largest, gets a place in the important party leaders' debates on TV in the run-up to the General Election.

During her visit to London for talks with Prime Minister David Cameron, the First Minister made clear that since the independence referendum - with membership of the SNP nearing 100,000 - the political landscape across the UK had changed significantly and for the broadcasters to ignore this was "not on".

"It would be completely unacceptable for the SNP and indeed the Greens and Plaid Cymru to be excluded from the General Elections debates," declared Ms Sturgeon.

Asked if the SNP were excluded by the broadcasters, would it consider taking legal action to ensure it was able to participate in them, she replied: "At this stage, I wouldn't rule anything out because the principle of being included and not being excluded is a very important one to us."

Leanne Wood, the Plaid Cymru leader, stressed how the Nationalist parties in Wales and Scotland provided an alternative to Westminster's austerity programme 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," she argued.

But Nick Clegg warned Ms Sturgeon and Ms Wood not to give David Cameron a reason to pull out of the debates.

He told his regular Whitehall press conference: "The debates are a good idea, a good innovation. What people want to see is the three main Westminster parties represented there; others will make their own claims to do so as well."

He then added: "It's very important, not least the Conservative Party is so reluctant to hold these TV debates altogether, 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 the way the public will welcome."

Later in response, Ms Sturgeon, at a briefing with Ms Wood and Natalie Bennett, the Green leader in England and Wales, dismissed as "pretty lame" Mr Clegg's warning that their objections could block the TV debates altogether.

"There is a democratic principle at stake here," she declared. "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 significant influence on what happens and who forms a government. We are articulating it as a principle that's important to our parties...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," added the FM.

Earlier this month, the BBC, ITV, Sky and Channel 4 announced joint plans to hold three debates in the six weeks before polling day.

One would be a head-to-head between Mr Cameron and Labour leader Ed Miliband; the second would involve these two leaders plus Mr Clegg while the third would also include Mr Farage, the Ukip leader.