The charges would mark the first enforcement action the government has taken against a major rating agency involving the worst financial crisis since the Great Depression.

S&P said Monday that the Justice Department had informed the rating agency that it intends to file a civil lawsuit focusing on S&P’s ratings of mortgage debt in 2007.

The action does not involve any criminal allegations. Critics have long complained about the government’s failure to bring criminal charges against any major Wall Street players involved in the financial crisis. Criminal charges would require a higher burden of proof and carry the threat of jail time.

If S&P is eventually found guilty of civil violations, it could face fines and limits on how it does business.

S&P denies any wrongdoing and says any lawsuit would be without merit.

A federal lawsuit would “disregard” the fact that S&P reviewed the same data on risky mortgages as U.S. government officials, who said publicly in 2007 that the problems in the subprime mortgage market appeared to be limited, the company said in a statement.

In the statement, S&P said it “deeply regrets” that its ratings on some securities “failed to fully anticipate the rapidly deteriorating conditions in the U.S. mortgage market during that tumultuous time.”

Justice Department spokeswoman Nanda Chitre declined to comment on the matter.

According to a report in the New York Times, the lawsuit will likely be brought this week after settlement talks between the Justice Department and S&P broke down last week. The talks collapsed over federal authorities’ insistence that a settlement involve at least $1 billion, the Times reported.

Judges have previously thrown out claims brought by investors against the rating agencies, on the grounds that their ratings amount to free speech protected by the First Amendment.

But that argument hasn’t always succeeded in cases involving investments like those in the expected S&P suit, according to research by The Brattle Group, a consulting firm. That’s because those ratings weren’t published widely, as most bond ratings are. As a result, several courts have ruled that those ratings do not enjoy free-speech protection.

S&P is a unit of New York-based McGraw-Hill Cos. McGraw-Hill’s stock plunged nearly 14 percent Monday after reports surfaced about the government’s expected lawsuit.

Moody’s Corp., the parent of Moody’s Investors Service, another rating agency, closed down nearly 11 percent. The two companies’ stocks suffered the biggest percentage drops in the S&P 500 index, which finished down slightly more than 1 percent.

S&P, Moody’s, and Fitch Ratings, the third major rating agency, have been blamed for helping fuel the financial crisis by assigning AAA ratings to trillions of dollars in risky securities backed by subprime mortgages. The securities collapsed once the housing bubble burst and home-loan delinquencies soared. Major U.S. banks absorbed tens of billions in losses.

The rating agencies are important arbiters of the creditworthiness of securities traded around the world. The grades they assign can affect a company’s ability to raise or borrow money and how much investors will pay for securities it issues.

The securities in the anticipated federal lawsuit are collateralized debt offerings. CDOs are investment vehicles that contain many underlying mortgage loans.

A CDO generally gains in value if borrowers repay. But a wave of defaults can cause them to tumble in value. Soured CDOs contributed to, and intensified, the financial crisis.

Critics have long argued that rating agencies have an inherent conflict of interest: They’re paid by the same companies whose products and credit they rate. The agencies have been accused of issuing unduly high ratings before the crisis, in part because of pressure from banks they desired as clients.