BERLIN — Plans by France and Germany to save the euro through closer European unity faced a serious challenge Tuesday from Britain, as deep divisions emerged between the 17 EU nations that use the euro and the 10 others that don’t.

Threatened by fears their joint currency may not survive, German Chancellor Angela Merkel and French President Nicolas Sarkozy had forcefully demanded changes to European Union treaties to tighten controls over spending and borrowing for all who use the embattled euro.

Their comments reinforced market expectations that EU leaders at a Friday summit would finally contain, through tighter financial rules, the two-year-old debt crisis that has engulfed the continent and threatens the entire global economy.

Enter David Cameron, the prime minister of Britain, a European nation that does not use the euro, who said he would be heading to the Brussels summit “to defend and promote British interests.”

“Eurozone countries do need to come together, do need to do more things together – if they choose to use the European treaty to do that, Britain will be insisting on some safeguards too,” he declared Tuesday. “As long as we get those, then that treaty can go ahead. If we can’t get those, it won’t.”

Cameron is the leader of Britain’s Conservative party, which resists transferring more powers to EU institutions in Brussels. Many party members have long wanted to ditch the EU altogether.

On the other hand, Cameron is wary of losing power with the 27-nation bloc if France and Germany create a tighter club of eurozone nations with tough rules for national budgets and automatic sanctions for those who stray.

The safeguards cited by Cameron included the importance of keeping a single EU market of some 500 million consumers and making sure that any eurozone moves don’t threaten London’s status as a global financial center.

Cameron recognized that Britain had a huge vested interested in seeing the eurozone resolve its problems, since a significant amount of British exports go to fellow EU nations.

“The most important British interest right now is to sort out the problem in the eurozone that is having the chilling effect on our economy,” he said.

Adding to the pressure, Standard & Poor’s warned it could downgrade 15 eurozone nations as well as Europe’s bailout fund if European leaders don’t act. And U.S. Treasury chief Timothy Geithner began a three-day European tour Tuesday to prod eurozone nations into action.