The ministry said Mansur, killed Monday by British Apache helicopter troops northeast of the provincial capital Lashgar Gah, was believed responsible for the deaths of two British service members and hundreds of civilians, CNN reported. Helmand is where British troops are based in Afghanistan.
The report said Mansur also was believed responsible for several other suicide attacks in which Afghan police and civilians died.
"U.K. forces conducted a successful precision strike against one of the most dangerous men in Helmand, and what we consider to be the most dangerous man in the central area around Lashkar Gah," Lt. Col. Nick Richardson was quoted as saying.
"The attacks he helped plan and execute have probably killed or wounded hundreds of people, and most of them have been either Afghan civilians or police."
The Times of London reported the British attack, in which several other militants and accomplices were also killed or injured, came early Monday morning, targeting an isolated area near Lashkar Gah.
British Defense Secretary John Hutton said the death of Mansur "is a significant blow to the Taliban," The Times reported.
"British forces will continue to work with international partners to improve security across Helmand to prevent the export of terror from Afghanistan to the U.K.," Hutton said.