NEW YORK — Until this week, the fate of Europe seemed to hang on the decisions of three power brokers – the president of France, the chancellor of Germany and the head of the European Central Bank.

Add a surprising fourth: Standard & Poor’s, the credit rating agency.

S&P ripped into American politicians last summer for failing to address long-term debt and stripped the United States of its top-flight credit rating. Now it is essentially warning Europe to fix its debt problem – or else.

Critics of S&P have questioned its credibility and relevance because it failed to foresee the collapse in the U.S. subprime mortgage market, which helped trigger the financial meltdown of 2008.

But what S&P says about the creditworthiness of European countries, or just about any other financial entity, still matters a great deal.

S&P rates companies and governments by their ability to repay debt. The higher the rating – AAA is the highest – the more investors trust them, and the less interest companies or governments have to pay to borrow money.

S&P threatened to lower its rating on 15 nations – even Germany, the most powerful economy in Europe – if their leaders don’t agree on a tough response to the European debt crisis.

Borrowing costs for European countries were little changed after S&P’s announcement, which came Monday night Europe time. But ratings cuts later could force countries to pay higher interest rates on the national bonds they issue to investors, creating a dangerous debt spiral and pushing them closer to default.

In a debt spiral, a country is forced to put aside an ever-larger share of its budget for interest. That leaves less for everything else, and the country has to borrow even more to make up the difference – or cut services, hurting the economy.

S&P’s warning drew angry responses from some European officials who are scrambling to contain the crisis, and outrage from critics who say S&P plays an outsize role in markets.

“It still has enormous power,” said Michael Lewitt, a former money manager who pilloried credit raters in his book “The Death of Capital.” ‘’I’m surprised it hasn’t been throttled back by now.”

The spotlight on S&P comes after it stripped the United States of an AAA rating for the first time in August, just after the prolonged debate over whether to raise the nation’s borrowing limit.

S&P normally focuses on the financial resources of a company or government to pay back what it owes. But in the U.S. downgrade, it called out Congress and the White House for their gridlock over how to fix the nation’s long-term debt problem.

Political considerations played a role in S&P’s European downgrade threat, too. In a conference call Tuesday, Moritz Kraemer, head of S&P’s sovereign ratings for Europe, said the agency is worried about paralysis in European decision-making.