Britain's economy relies on trade and financial services.
The free flow of goods and services with the European continent has been a boon, but more and more Britons see the European Union as an unwelcome infringement on their sovereignty.
That has pushed Prime Minister Cameron to promise a re-negotiation of Britain's ties to the EU and then a referendum within five years, if he is re-elected in the middle of the process.
"We've been very clear about what we want to see changed.
There are a whole series of areas, social legislation, employment legislation, environmental legislation , where Europe has gone far too far," he said.
Cameron said he wants to preserve the single market but avoid some of its regulations, an approach that analyst Stephen Tindale, at the Center for European Reform, says won't work.
"His plan is impractical if it works because there would be lots of non-tariff barriers, different regulations, different standards, so products couldn't be as freely traded within Europe as they are at the moment," he said.
That would seem bad for business but, on Thursday, 56 British business leaders endorsed the prime minister's plan, including heads of the London stock exchange and one of the country's major banks.
They decried what they called "ever more burdens from Brussels."
But Tindale warns the changes those executives and the prime minister envision are not likely to be accepted by other European leaders.
"The other EU members would ultimately let him walk if his shopping list of demands for repatriation of powers are too great," he said.
European officials have indicated as much, including the French government spokeswoman.
"Being a member of the European Union brings with it a certain number of obligations, and the Europe we believe in and we live in is a solidarity pact which applies to all member states.
Otherwise it's not solidarity," said Najat Vallaud-Belkacem.
But Britain has never been interested in as much solidarity as many other EU members.
Years ago, it opted out of two key aspects of the Union - the common currency and the open borders agreement.
Europe expert Iain Begg at the London School of Economics says while the euro crisis is pushing much of the continent toward closer integration, Britain wants less.
"I can see Europe going in one direction, which is seeking to integrate more, Britain becoming increasingly uneasy with this federalizing tendency, and therefore divorce is the only outcome," he said.
Prime Minister Cameron and his supporters don't want a divorce, just a better relationship.
But the plan to negotiate with the 26 other EU members and then hold a referendum is fraught with uncertainty, and, experts say, could lead Britain where Cameron says he doesn't want it to go - out of the European Union.
He is an adventurer in financial field.
No infringement will be permitted.
The question was decided by referendum.
The new legislation is highy discriminatory.
He ruled out the impractical option.