BASRA, Iraq — Prime Minister Brown announced yesterday that he was slashing the remaining British contingent in Iraq by nearly 20%. A beleaguered Iraqi leader said his own forces would be ready to take up the slack in the country’s oil-rich southernmost province in two months.

Mr. Brown’s one-day, unannounced swing through Iraq comes as American military officials are concerned that the reduced British presence in the South could open security gaps along key supply and transit routes to Kuwait.

The roadways are a vital lifeline for American forces. And everything that the Americans can’t fly out of the country when they eventually leave must make the long and potentially dangerous road journey to Kuwait through Basra province.

Mr. Brown is said to be contemplating early elections in Britain, where the war is deeply unpopular. He arrived in Baghdad midmorning and went straight into a meeting with his Iraqi counterpart, Prime Minister al-Maliki.

“We are prepared to take over security of Basra within two months and we will,” Mr. Maliki said after the meeting in his Green Zone office. “Basra will be one of the provinces where Iraqi forces will completely take over security.”

Mr. Brown confirmed Mr. Maliki’s plans and said, “as we move to overwatch, we can move down to 4,500” troops from the current level of 5,500. He promised to bring the 1,000 troops home by Christmas.

Last month, British troops vacated their last remaining base in downtown Basra, Iraq’s second-largest city, 340 miles southeast of Baghdad. The British force now is based mainly at an air base on the fringes of Basra city inside Basra province.

The British leader said any further decision on troop withdrawals would be made next year. His visit overshadowed the opposition Conservative Party conference in Blackpool, England.