President Bush signed into law on Sunday legislation that broadly expanded the government’s authority to eavesdrop on the international telephone calls and e-mail messages of American citizens without warrants. Congressional aides and others familiar with the details of the law said that its impact went far beyond the small fixes that administration officials had said were needed to gather information about foreign terrorists.

They said seemingly subtle changes in legislative language would sharply alter the legal limits on the government’s ability to monitor millions of phone calls and e-mail messages going in and out of the United States.

They also said that the new law for the first time provided a legal framework for much of the surveillance without warrants that was being conducted in secret by the National Security Agency and outside the Foreign Intelligence Surveillance Act, the 1978 law that is supposed to regulate the way the government can listen to the private communications of American citizens. “This more or less legalises the NSA programme,” said Kate Martin, director of the Center for National Security Studies in Washington, who has studied the new legislation.

Previously, the government needed search warrants approved by a special intelligence court to eavesdrop on telephone conversations, e-mail messages and other electronic communications between individuals inside the United States and people overseas, if the government conducted the surveillance inside the US.

Today, most international telephone conversations to and from the US are conducted over fibre-optic cables, and the most efficient way for the government to eavesdrop on them is to latch on to giant telecommunications switches located in the US.

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