BAGHDAD, Monday, Sept. 3 — The British Army began withdrawing from its last base in Basra’s city center early Monday, a move that will leave Iraq’s second-largest city without foreign forces for the first time since the American-led invasion in 2003.

Basra residents reported overnight that they saw British military trucks accompanied by armored vehicles and helicopters leaving the base, Basra Palace, beside the Shatt al Arab waterway, heading for their airport headquarters miles outside the city, the oil-industry hub of southern Iraq.

While the British exit from Basra had been widely anticipated, the British government has given no timetable for the eventual withdrawal of British troops from Iraq.

British officials would not publicly confirm the withdrawal of the 500 or so remaining troops in the palace, apparently fearing that the disclosure would compromise their security. However, the Ministry of Defense said through a spokesman in London: “There is an operation under way. It is ongoing now and will continue to be ongoing for hours, possibly days.”