Britain has announced the biggest restructuring of its armed forces in a generation, with steep personnel cuts, base closures and the retirement of many ships, planes and tanks.

British officials say the changes are needed to modernize the armed forces in an era of terrorist threats and technological advances.

Over the next four years, about 15,000 uniformed personnel will be dropped from the ranks, and 10,000 civilian defense employees will lose their jobs.

The Royal Air Force will take the biggest hit, with the jobs of nearly one in every seven airmen to be cut.

Also facing elimination are four infantry battalions, seven tank squadrons, six artillery batteries, four air force squadrons and a total six of Royal Navy destroyers and frigates.

Prime Minister Tony Blair told parliament the cuts are not as dramatic as they appear.

"Defense spending under this government is rising," he said. "Yes, of course, there will be changes in the way we spend the defense budget. That is sensible. As the world changes it is important that we make changes to the way we spend the money in relation to defense. There are no reductions in front-line troops that are anticipated by this review today. But what is important is to make sure that we use the money we're spending, record amounts of money on defense, in the most sensible way."

Critics say the reductions are too deep at a time when British forces are deployed in Iraq, Afghanistan, Bosnia, Kosovo and Sierra Leone. Among the critics is the defense spokesman of the opposition Conservative Party, Nick Soames.

"The armed forces are undermanned and overstretched," said Mr. Soames. "They've got too much on their plate. The government is going to give them some new money. But they are also making very substantial cuts."

Defense analysts point out that specialized units needed to fight terrorism, such as the Royal Marines and defense intelligence, have been spared.

Also, defense ministry officials say Britain's troop commitment to Iraq will not be affected.