WASHINGTON — President Bush, for the second time in three years, has authorized the Marine Corps to recall reservists for mandatory duty in Iraq, Afghanistan, and the Horn of Africa, an official said yesterday.

Mr. Bush’s authorization gives the Marines permission to recall as many as 2,500 people for 12- to 18-month stints to fill volunteer shortages in areas such as military police, communications specialists, truck drivers, and intelligence, Colonel Al Stratton, a Marine Corps personnel official, said.

The Marines are facing a shortage of about 1,200 volunteers for certain military specialties, Colonel Stratton told reporters yesterday. “The numbers have been consistently going down across the board,” he said.

The first deployment orders for Marines from the Individual Ready Reserve, which has about 35,000 available people, will not go out for several weeks, and the individuals mobilized will not be required to report until early next year, Colonel Stratton said.

The number of people actually called up from the pool of eligible personnel will be small, he said.

In 2003, Mr. Bush authorized the Marines to recall 7,500 Marine reservists for the invasion of Iraq, but only about 2,000 were eventually given orders. About 24,100 active duty and reserve Marines are in Iraq, Kuwait, Afghanistan, and the Horn of Africa today. America has about 138,000 troops in Iraq.

Meanwhile, Britain plans to cut the number of troops it has in Iraq in half by the middle of 2007, handing over control for security earlier than previously forecast, two government officials with knowledge of the plan said yesterday.

The plan would reduce the size of the British force patrolling Basra and other parts of southern Iraq to between 3,000 and 4,000. Britain currently has 7,000 troops in the nation and would begin the withdrawal in six months, the officials said in London.

Prime Minister Blair in July said a “significant” drawdown of British troop levels would begin by the end of 2007. Yesterday’s announcement is the first to show the scope and timing of the plan and reflects efforts to build up Iraqi forces capable of battling a growing insurgency in the country.

“We’ve done a lot of training for the Iraqi army over the past two years, and they’re up to the job,” Louise Heywood, the head of the British armed forces program at the Royal United Services Institute, a London-based military researcher, said. “They’ve become a force that’s capable of standing up to the insurgents.”

The pace of withdrawal will be governed by the ability of Iraq’s government to take control of regions currently run by the British. At least 115 British soldiers have been killed in Iraq since the American-led invasion in 2003.

Mr. Blair’s popularity has suffered since the invasion, dragging support for Britain’s ruling Labour Party to its lowest in 19 years, an ICM Ltd. survey published yesterday showed.

Labour had the support of 31% of voters, down 4 points from a month ago. Liberal Democrats, who opposed the war, gained 5 points to 22%, and the Conservative Party had 40%.

Prime Minister al-Maliki said in May that he wants his government’s own troops to be providing security in all except two of the nation’s 18 provinces by the end of the year. The British military handed over control of the Muthanna province to Iraq in June, and aims to hand over control of Basra province in the first quarter of 2007.

Britain will then consolidate forces to a single base in the city of Basra, in southern Iraq, one of the officials, a senior military commander speaking on condition of anonymity, said. The troops will remain to support Iraqi forces, he said.

Once the reduction is complete, British forces will be concentrated in Basra for at least three years while American troops remain in Iraq. British soldiers will not be sent to Baghdad, the officials said.

According to a senior defense official, also speaking on condition of anonymity, it will be up to Iraqi authorities to decide when control is handed over.