The US Justice Department said on Friday that Microsoft's introduction of a new release of its Internet Explorer Web browser has weakened the company's defence in its antitrust trial.

During the trial, which is in recess, government lawyers contended that Microsoft bundled its Web browser with its Windows operating system in order to ensure that customers would use Internet Explorer (IE) as their browser. Microsoft claimed that its browser had become an integral part of the operating system and could not be separated without affecting functionality.

But the ability of IE5, officially released on Thursday, to run on several operating systems shows it is a separate product from Windows, a Justice Department spokeswoman said.

"This (browser) undermines the primary argument they were making at the trial," she said. The spokeswoman, who requested anonymity, echoed statements she said were made by David Boies, the government's lead lawyer in the trial.

A critic of Microsoft agreed with the government's claim.

"What happened to Microsoft's contention that consumers couldn't enjoy the benefits of the browser without it being integrated into the operating system?" asked Mike Pettit, executive director of ProComp, a Washington, DC-based association of companies and trade groups interested in information technology issues.

"Nearly every Microsoft executive and witness has testified under oath that the browser is an integral part of Window's operating system and can not be separated," Pettit said. "But by making the new browser available for different operating systems and separate from Windows, Microsoft is making two concessions which are obvious to the rest of the world, but stunning for that company: Internet Explorer is a separate application, not necessarily part of the Windows operating system, and consumers can benefit from using the browser itself, without integration."

A Microsoft spokesman denied the new browser is a separate product and said the company will prove it if necessary at trial.

"The Internet Explorer 5 is an upgrade of the Windows operating system," said Microsoft spokesman Jim Cullinan. "Upgrades are a normal part of the industry."

A Stanford Law School professor questioned whether Microsoft can be held accountable for an innovation completed after the antitrust lawsuit was filed.

"This illustrates the difficulty of matching the judicial process to the speed of technology development," said Carey Heckman, who is also the co-director of the Stanford Law and Technology Policy Centre.

The government may find it hard to prove IE5 is a totally separate, rather than an upgraded, product, Heckman said. "The browser is still part of the operating system," he said.

Whatever the impact on the trial, the introduction of IE5 shows Microsoft has not been cowed by the litigation from issuing new products, Heckman added. "They are going ahead full steam," he said.

Meanwhile, spokespeople for both the Department of Justice and Microsoft said they had no information on a report today that the trial recess would be extended to May. A status hearing is scheduled for March 31 at which point the judge will decide when trial will resume, which will be no earlier than April 12, Cullinan and the DoJ spokeswoman said.