By William Schomberg and Marc Jones
LONDON (Reuters) - British Prime Minister David Cameron took on euro-skeptic critics in his own party on Thursday, saying he was able to negotiate a better deal with Brussels and it was wrong to say Britain should leave the European Union.
He described as pessimists those who argue Britain should leave the bloc and say there is no prospect of reforming the EU.
"I think they are wrong ... I think it is possible to change and reform this organization," Cameron told an investment conference.
Cameron came under renewed pressure from within his Conservative party this week when former finance minister Nigel Lawson said a plan to renegotiate Britain's commitments to the EU, before an expected membership referendum in 2017, was doomed to fail and the country should leave the bloc.
London mayor Boris Johnson, speaking at the same conference, said the party should rally around Cameron and his plan.
But Johnson, who is believed to harbor ambitions to succeed Cameron as leader of the Conservative party, also said Britain should be ready to "walk away" from the EU if it fails in its renegotiation bid.
"This is not for Britain the existential question it was ... when we joined at the height of the Cold War," he said. "We now live in a globalised economy where the real growth markets are to be found outside the European Union."
Membership of the EU has been a divisive issue for the Conservatives for decades and Cameron hopes the referendum will settle it once and for all. It depends not only on securing more favorable EU membership terms but also on the Conservatives forming Britain's next government after elections in 2015.
Cameron underscored his determination to keep on narrowing Britain's budget deficit at a "sensible and measured pace" and to help push for new trade deals between the EU and the United States and Canada.
He also said he would continue to defend Britain's financial services industry against some European measures such as a planned financial transaction tax which has been agreed by most countries in the euro zone and would affect the City of London.
"We shouldn't spend our time in politics endlessly bashing banks and financial institutions. If you want the economy to recover and if you want the economy to grow, you have got to play to your strengths," he said.
(Editing by Louise Ireland and Sonya Hepinstall)