Many of the largest United States corporations, including big banks now receiving federal bailout money, operate scores of subsidiaries in offshore tax havens that may let them evade or defer their tax bills, according to a government study released Friday.

The study, by the Government Accountability Office, singled out Citigroup as having 427 subsidiaries in offshore havens like the Cayman Islands, British Virgin Islands and Switzerland. Bank of America has 115 subsidiaries in offshore havens, while Morgan Stanley has 273, the report said.

Bank of America received an additional $20 billion in government aid on Friday, on top of a previous $25 billion, and a federal promise to absorb nearly $98 billion in soured mortgage-related securities. Citigroup is expected to get $50 billion.

The 63-page report drew unusually revealing criticism from the Treasury Department. Michael F. Mundaca, the deputy assistant secretary for international tax affairs, wrote in a letter included in the report that because there was no universal definition for an offshore tax haven, any list of havens “is likely to be regarded as a blacklist and may be used as the basis for the imposition of sanctions or other negative measures” that would “inappropriately negatively affect our economic and other relations with listed countries.”