Conservative legislators have urged Prime Minister David Cameron to ensure he wins adequate protection for Britain's financial services industry at a European Union summit designed to defuse the continent's debt crisis.

Cameron, who will arrive in Brussels for the summit later on Thursday, is walking a fine line: he risks angering his own party if he backs measures to rescue the euro zone economy that involve ceding more powers to European institutions.

Divisions over Europe tore apart the Conservative Party last time it was in power in the 1990s and Cameron is painfully aware of how toxic the issue can be.

European Union proposals pose a grave threat to Britain's financial services industry, said the letter, signed by more than 20 Conservative MPs and published in the Daily Telegraph.

It is imperative that the Government fights our corner by arguing either for a new EU protocol or a Britain-specific legal safeguard, it added.

Without strong action, the present drift seriously threatens both British jobs and Exchequer (tax) revenues, it added.

The letter adds to pressure on Cameron to return from the summit on Friday with something to reassure his restive right-wing who see the current European crisis as a unique opportunity to reclaim powers from Brussels.

Cameron calls himself a eurosceptic but says that the priority now must be rescuing the economies in the euro zone, Britain's biggest trading partner.

He promised to secure safeguards for Britain's banking industry during a fractious session of parliament on Wednesday when members of his own party urged him to show some traditional British bulldog spirit at the summit.

Britain is the largest of the 10 countries that are part of the EU but not in the single currency.

It is particularly concerned about Franco-German plans for a financial transactions tax which it says would hit London hard. Despite efforts to rebalance the British economy, financial services still account for more than 10 percent of the economy.

Cameron's position is made yet more difficult by the fact that he governs in coalition with the smaller, pro-Europe Liberal Democrats.

The prime minister has played down talk of a referendum on changes to the EU Treaty, disappointing party members who want Britons to have a vote to define their ties with the bloc. Britain joined in 1973 but has long had an ambivalent relationship with the rest of Europe and a distrust of deeper EU integration.

Northern Ireland Secretary Owen Paterson, a Conservative, added to Cameron's discomfort by saying that a referendum was inevitable if a more integrated euro zone was created.

This isn't going to happen immediately because these negotiations are going to take some months, Paterson told the Spectator magazine.

But I think down the road that is inevitable.

(Editing by Mark Trevelyan)