David Cameron has said he wants to encourage more responsible lending and action to help customers at risk of becoming fresh victims of the credit crunch fallout.

The Conservative leader said the repercussions of the crisis sparked by bad debt in the US were being felt increasingly widely, and 1.4 million people due to come off fixed-rate mortgages this year would be paying on average £200 more every month.

Speaking on BBC Radio 4's Today programme he said he would be telling a meeting of UK mortgage lenders to help borrowers cope with the new situation.

"That could be staggering the increase in mortgage payments when they come in, it may mean switching some customers to interest-only mortgages," he said.

"What I will be saying to them is that they must work together and make sure that they adopt best practice."

However he insisted that more regulation was not the answer to the problem of irresponsible lending.

"I believe in something called social responsibility rather than always seeking a regulatory answer," said the Tory leader.

"We should be asking banks and building societies - who are responsible organisations - to work together to act responsibly to learn from each other and make sure they do everything they can."

Cameron also warned that it would be a "monumental failure" if the government was forced to nationalise Northern Rock.

The Tory leader insisted it "would not be right" for the taxpayer to take charge of the troubled bank and become responsible for billions of pounds in mortgages.

"I don't support nationalisation [of Northern Rock], I don't think it would be right for government to spend even more taxpayers' money nationalising a bank and suddenly becoming responsible for £100bn of mortgage lending," he said.

"What we want is a private sale, but I suspect that the government is running out of time and running out of money and it may well be that they need to go down the path of nationalisation. If it does, it will be a monumental failure."

However, Liberal Democrat acting leader Vince Cable insisted that bringing Northern Rock under state ownership was now the "least worst" option, and could save the public purse billions.