The US government is expected to file civil charges against Standard & Poor's Ratings Services, alleging that it fraudulently gave high ratings to mortgage debt that later plunged in value and helped fuel the 2008 financial crisis.
The charges would mark the first enforcement action the government has taken against a major rating agency involving the financial crisis.
S&P said on Monday that the Justice Department had informed it 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.
S&P denies any wrongdoing and says any lawsuit would be without merit.
A lawsuit would "disregard" the fact that S&P reviewed the same data on risky mortgages as the rest of the market and US government officials, who publicly said 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 US mortgage market during that tumultuous time".
Justice Department spokeswoman Nanda Chitre declined to comment on the matter.
S&P is a unit of New York-based McGraw-Hill Cos.
The company's stock tumbled 13 per cent in heavy trading on Monday amid a broader market decline.
S&P and the other two major agencies, Moody's Investors Service and Fitch Ratings, have been blamed for helping fuel the crisis by assigning AAA ratings to trillions of dollars in risky securities backed by subprime mortgages.
The securities later collapsed in value once the housing market bubble burst and home-loan delinquencies soared.
Major US banks absorbed tens of billions of dollars in losses.
The rating agencies are crucial 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 collateralised 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 because of pressure from banks they desired as clients.