WASHINGTON — In another sign that pressure is mounting to resume underground nuclear testing in Nevada upwind from Utah, a British think tank now worries aloud that Britain may want to join such testing to modernize its aging nuclear stockpile.

The British American Security Information Council (BASIC) is calling on Parliament and Congress to look more closely at a proposed 10-year extension of an agreement to share nuclear data for "mutual defense purposes."

Nigel Chamberlain, BASIC's nuclear analyst, said renewing it without close questioning and outlining of goals may suggest to the world that Britain and America are working on new and better nuclear weapons while they are trying to prevent other countries from developing any at all.

"It is probably not the most effective for them to tell others they ought not to acquire nuclear weapons, but we are going on for another 10 years (of study on new weapons) no matter what you are doing," he told the Deseret Morning News.

In short, he worries it could lead to more nuclear arms competition and would work against the Nuclear Proliferation Treaty designed to stop that. His group and allies even plan legal challenges to the Mutual Defense Agreement on those grounds.

Steve Erickson, spokesman for the Utah-based Citizens Education Project anti-testing group, notes that the Nevada Test Site was home to 24 joint British-U.S. underground nuclear tests between 1962 and 1991. Before that, Britain conducted 21 open-air nuclear tests, primarily in Australia and the Pacific.

Erickson notes that Britain and America jointly developed and deployed warheads for Trident missiles launched from their submarines. BASIC issued a report this month outlining numerous signs from British leaders that they are considering updating their Trident warheads, and may push for permission after the next national elections.

"I haven't seen anything about America wanting to update its Tridents. We have talked more about developing mini-nukes and bunker-busters," Erickson said. "But if the British want to update the Trident, I would assume it would be a joint effort since it was developed jointly."

He added that the British agenda "would be another pressure to resume testing. The United States and Britain have never deployed a new system without testing it first. And that does not mean just one test. Historically, it means a series of several tests."

Chamberlain's group is concerned that with the Trident nearing the end of its intended life span, British scientists — if the Mutual Defense Agreement is extended — may tap into the development of smaller more usable weapons such as the mininukes and bunker-busters receiving early study now by the Bush administration.