WASHINGTON - Researchers uncovered a serious flaw last year in the underlying technology for nearly all Internet traffic, a discovery that led to an urgent and secretive international effort to prevent global disruptions of Web surfing, e-mails and instant messages.

The British government announced the vulnerability in core Internet technology yesterday. Experts said the flaw could allow hackers to knock computers off-line and broadly disrupt routers, vital devices that coordinate the flow of data among distant groups of computers.

"Exploitation of this vulnerability could have affected the glue that holds the Internet together," said Roger Cumming, director for England's National Infrastructure Security Coordination Centre.

The Homeland Security Department issued an alert hours later that such attacks "could affect a large segment of the Internet community." The department said normal Internet operations probably would resume after such attacks stopped.

The risk is similar to Internet users "running naked through the jungle, which didn't matter until somebody released some tigers," said Paul Vixie of the Internet Systems Consortium Inc.

"It's a significant risk," Vixie said. "The larger Internet providers are jumping on this big time. It's really important this ... gets fixed before the bad guys start exploiting it for fun and recognition."

The flaw, which affects the Internet's transmission control protocol, was discovered last year by Paul A. Watson, a computer researcher in Milwaukee. Watson said he identified a method to reliably trick personal computers and routers into shutting down electronic conversations by resetting the machines remotely.

Routers continually exchange important updates about the most efficient traffic routes between large networks. Continued successful attacks against routers can cause them to go into a standby mode, known as dampening, that can persist for hours.

Experts had thought that such attacks would take between four years and 142 years to succeed because they require guessing a rotating number from among about 4 billion possible combinations. Watson said he could guess the proper number within seconds with as few as four attempts.

In recent weeks, some U.S. government agencies and companies operating the most important digital pipelines have fortified their systems.

The White House has expressed concerns, especially about risks to crucial Internet routers, because attacks against them could disrupt the Web.

"Any flaw to a fundamental protocol would raise significant concern and require significant attention by the folks who run the major infrastructures of the Internet," said Amit Yoran, the government's cyber security chief.

The public announcement coincides with a presentation Watson is to make tomorrow at a Web security conference in Vancouver, British Columbia, where, he said, he will disclose full details of his research.

Watson predicted that hackers would understand how to begin launching attacks "within five minutes of walking out of that meeting."