LONDON - Prime Minister Tony Blair warned British troops yesterday to prepare for action against Iraq if President Saddam Hussein failed to comply with international weapons inspections demanded by the United Nations.

In a Christmas message on the British Forces Broadcasting Service, Blair apologized for the uncertainty the troops face but said it was unavoidable under the circumstances.

"At the moment, we simply don't know whether Iraq will be found in breach of the United Nations resolution," he said.

Britain, Washington's principal military ally, is preparing to send as many as 20,000 troops along with tanks and other materiel to the region in the coming weeks, according to the Ministry of Defense.

Ships are being chartered to ferry equipment, and the aircraft carrier Ark Royal is to lead a six-ship task force that will head to the Persian Gulf next month.

The 6,500 reservists who are expected to be called up in the event of war have received letters saying that their service may be required, and their employers have been notified of their potential absence.

In disclosing the plans this week, military chiefs said the actions did not mean war was inevitable, calling them necessary steps for preparedness.

Blair has had to walk a fine line between maintaining his strong support for President Bush and calming anxieties elsewhere in Europe. Even Labor Party members of Parliament and members of his Cabinet have expressed concerns.

Blair's critics portray him as slavishly bound to Bush and question whether he has had the moderating effect on Bush and the more hawkish members of the Bush administration that his supporters have credited him with.

In Berlin, a spokesman for Chancellor Gerhard Schroeder said Germany was considering a new request from the United States to supply troops to guard American military bases there, starting at the end of next month.

The spokesman said Germany had agreed to provide extra security but had not decided whether to deploy soldiers or other security forces.

The request apparently reflected concerns that American facilities in Germany could become targets for terrorism in the event of war with Iraq.

Of the possible decision to send British troops into action, Blair said, "These are the hardest decisions because you are aware that you are putting people's lives at risk, and that's why we should never undertake conflict unless we have exhausted all other options and possibilities."