REPORTING FROM LONDON — The leaders of 23 European countries agreed early Friday to bind their nations together in a pact to limit government spending and borrowing but failed to persuade Britain to sign up, dealing a blow to European unity.

The agreement came after marathon talks in Brussels at a two-day summit billed as the last chance to rescue the euro from the debt crisis that threatens the single currency’s survival. Leaders from 23 European Union countries said they would harmonize their fiscal policies by putting caps on government debt and deficits and slapping sanctions on nations that break the rules.

But four of the EU’s 27 member states declined to join the new treaty, chief among them Britain, whose leader, Prime Minister David Cameron, said it would run counter to British national interests. Cameron had insisted beforehand that he would sign up only if he could win special exemptions for London’s crucial financial services industry from European regulations, such as a proposed financial transactions tax. The other countries, led by France and Germany, refused to grant that concession.

Hungary, Sweden and the Czech Republic also decided to stay out of the treaty, at least for now. Like Britain, those three countries maintain their own currencies and do not use the euro.

[Updated at 6:44 a.m. Dec. 9: A statement issued later by the Eurozone suggested that Hungary, Sweden and the Czech Republic were open to signing the new treaty after more national deliberation, leaving Britain as the lone EU country to reject the pact completely.]

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