UK withdrawal from the European Union would not be as "cataclysmic" for British jobs as supporters of membership claim, Boris Johnson has said.

The London mayor was speaking amid growing pressure on David Cameron over Europe, with Tory grandees including Lord Lawson and Michael Portillo advocating withdrawal, and backbenchers hoping to force a Commons vote next week in protest against the prime minister's failure to table legislation to pave the way for a referendum.

Johnson said on Friday he did not back Lawson's call for the UK to leave the EU, but said Britain must be ready to do so if Cameron's plan to renegotiate its membership following the 2015 general election fails.

The mayor told BBC Radio 4's Today programme: "My position is that I think the prime minister is completely right. What we need to do is to say to our friends and partners: 'Listen chums, this thing isn't working for either of us. Your eurozone is causing all sorts of misery, plus the on-costs, the non-wage costs, of the way the EU has been running and has legislated and regulated over the last 20-30 years are making the whole area completely uncompetitive. We need a renegotiation. We need a look at the way the thing is managed.'"

He added: "We should be prepared to pull out. That goes without saying. You can't go into a negotiation like that without being willing to walk away."

Johnson insisted that withdrawal would not be as damaging to the UK economy as pro-Europeans claim, saying: "I don't think it would be anything like as cataclysmic as is being sometimes pretended.

"I don't think we would lose millions of jobs. I think the economic benefits and disbenefits [sic] are now much more balanced."