LONDON (REUTERS) - British Prime Minister David Cameron took on critics in his own Conservative party on Thursday, saying it would be wrong for Britain to leave the European Union.

Some pessimists "say there is no prospect of reforming the European Union, you simply have to leave. I think they are wrong ... I think it is possible to change and reform this organisation," Cameron told an investment conference.

Cameron came under renewed pressure from EU sceptics this week when former finance minister Nigel Lawson said the prime minister's plan to renegotiate Britain's commitments to the EU before a planned membership referendum in 2017 were doomed to fail and the country should leave the bloc.

Cameron used his speech on Thursday to underscore 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," Cameron said.