LONDON (Reuters) - Britain should leave the European Union because Prime Minister David Cameron's plan to claw back powers from Brussels is doomed, a former senior minister said on Tuesday, stirring divisions that threaten his party's re-election hopes.
The intervention of Nigel Lawson, a heavyweight in the Conservative party who served as Margaret Thatcher's Chancellor for six years, piles pressure on Cameron days after his party suffered from a surge in support for the anti-EU UK Independence Party (UKIP) in local elections.
Cameron came to power in a coalition government in 2010 with a plea to his party to "stop banging on about Europe", an issue that has divided the Conservatives for decades and helped bring down two of his predecessors, Thatcher and John Major.
But, despite Lawson's comments highlighting that deep divisions over Europe persisted within his party, Cameron said it had been "good day" for his pledge to hold an "in-out" referendum if he is re-elected in 2015.
"I welcome the attention that is being placed on this key pledge," he told reporters. "I want to give people a proper choice between Britain remaining in a reformed European Union or leaving that European Union."
Lawson, Chancellor from 1983 to 1989 and who now sits in parliament's upper chamber, is the most senior member of Cameron's Conservative Party to call for Britain to withdraw from the EU.
Cameron's attempts to repatriate powers from Brussels would probably only secure "inconsequential" results, Lawson said, echoing warnings from France and Germany.
Britain would be better off outside a 27-nation bloc that has become a "bureaucratic monstrosity", he wrote in Tuesday's Times newspaper.
"I strongly suspect that there would be a positive economic advantage to the UK in leaving the single market," Lawson wrote in an article that stirred memories of a Conservative civil war over Europe that raged for large parts of the 1980s and 1990s.
"In my judgment the economic gains would substantially outweigh the costs."
Lawson, who voted to stay inside the EU's forerunner in Britain's last referendum on Europe in 1975, said the euro zone debt crisis had fundamentally changed the EU and he would choose to leave if another vote was held.
More than 500 business leaders backed Cameron's renegotiation policy in April, saying a new, looser relationship with Europe would boost the British economy.
Others fear the referendum pledge has created years of uncertainty that will deter foreign investment in Britain and upset allies in the EU, Britain's biggest trading partner.
Cameron, trailing in the polls by around 10 points, supports Britain's continued membership of a club it joined at the third attempt in 1973. The latest YouGov poll found 43 percent of voters would leave the EU and 35 percent would stay in.
Editing by Guy Faulconbridge and Alison Williams