NICK Clegg has warned the country against allowing a "rag-tag mob" of MPs from the SNP or Ukip to hold the balance of power after the General Election, insisting only the Liberal Democrats could form part of a government that would stand up for modern Britain and liberal values.

In his keynote speech at the party's spring conference in Liverpool, the Deputy Prime Minister also sought to bolster activists' morale in the run-up to May 7 by insisting the Lib Dems had been challenged in government and had "passed the test".

Stressing how his party had held its nerve in face of deep criticism, he claimed the Lib Dems had proved coalition government "can be a very British way of getting things done".

"We challenged the power of the vested interests on the Left and Right of the British Establishment and we felt their wrath in return. And we are still standing."

It was because of the third party's resilience that Britain had a strong and stable government and a strong and stable economy.

And Mr Clegg argued it was because of that resilience the Lib Dems would defy the odds and win again at the election.

"So when people tell you we can't, tell them where to go. I have a message for all those who are writing us off once again: the Liberal Democrats are here to stay."

After Vince Cable, the Business Secretary, said it was "almost inconceivable" the Liberal Democrats would go into a governing partnership with the SNP, Mr Clegg went further.

"Let me be clear," he told conference, "just like we would not put Ukip in charge of Europe, we are not going to put the SNP in charge of Britain; a country they want to rip apart. It's just not going to happen."

Insisting only his party could keep the country anchored in the centre ground and on the right economic track, promising "light at the end of the tunnel" after years of austerity, the party leader said: "What will Britain become if Cameron's Conservatives or Miliband's Labour spend the next five years begging for votes from that rag-tag mob of Nationalists, populists and special interests?

"Not one of them will stand up for the moderate majority. Not one of them will keep Britain united."

Mr Clegg's rallying message to activists came as a row developed between two of the party's most popular figures over its record in office.

Paddy Ashdown, the Lib Dem election co-ordinator, publicly slapped down Tim Farron, the former party President, who is seen as a potential successor to Mr Clegg, for criticising their achievements in coalition; he said the Lib Dems should be rated only two out of 10 for their performance in the Coalition and warned that the party could suffer for a generation as a result of the decision to go into government with the Conservatives.

Lord Ashdown noted: "Tim's a very able guy but at the moment judgement is not his strong suit."