WASHINGTON– The Supreme Court gave prosecutors a powerful tool yesterday to attack the spread of child pornography online, ruling that people who send messages over the computer offering or seeking sexual images of children can be sent to prison, even when no such pornography exists.

The 7-2 ruling, which upheld a five-year-old law, rejected the claim that such messages are protected as free speech.

“This will be a big help,” said Patrick Trueman, a Virginia lawyer who led the U.S. Justice Department’s anti-obscenity unit during the Reagan and George H.W. Bush administrations. “You don’t have to prove the underlying material is illegal. The criminal offense is the speech that offers to sell or trade in illegal material.”

Yesterday’s ruling goes a long way toward overturning the effect of a 2002 decision that shielded computer-generated pornography. The justices said then that because no real children were involved in producing these images, this material deserved protection as a form of free speech.

Prosecutors then faced a practical problem. How could they prove that realistic images of child sex scenes involved actual children? Defendants could assert they were “virtual” pornography.

In response, Congress passed a law in 2003 making it crime to exchange online messages about “any material or purported material” that would cause “another to believe” it depicts a minor engaged in sex, whether “actual or simulated.” Violations call for at least five years and as many as 20 years in prison.

The fate of this law remained in doubt until yesterday because of legal challenges. Two years ago, a U.S. appeals court in Atlanta ruled the law unconstitutional because it was overly broad. It could apply, in theory, to a grandfather who sends an e-mail entitled “Good pics of kids in bed” to describe a photo of sleeping toddlers, the appellate judges said. Other critics said the law could ensnare people who tout a Hollywood movie that contains simulated sex scenes.

In yesterday’s opinion, the Supreme Court dismissed those objections as “fanciful hypotheticals” and upheld the law.

“Child pornography harms and debases the most defenseless of our citizens,” Justice Antonin Scalia said. “We hold that offers to provide or requests to obtain child pornography are categorically excluded from the First Amendment.”

Justice Scalia knocked down each of the objections to the new measure.

No prosecutor or jury would believe that a grandfather was offering children engaged in “sexually explicit conduct” when he sends an e-mail describing “kids in bed,” Scalia said. “The statute has no application … where the material at issue is a harmless picture of a child in a bathtub,” he added.