The identity of the soldier has not been released, although it has been confirmed that he was a member of 7th armoured brigade.
Three other soldiers were injured in the blast in Um Qasr.
The fatality comes after another British soldier from the 1st Battalion The Highlanders -- also part of 7th Armoured Brigade -- was killed while on patrol in the southeastern Maysan province on Monday.
Some British newspapers named the soldier as Lance Corporal Alan Douglas, a 22-year-old from Aberdeen, northeast Scotland, who reportedly had only been in Iraq for about seven weeks.
"Three other soldiers were injured, one seriously, in the same incident and are receiving medical treatment at Shaibah medical facility," said a spokeswoman from the Ministry of Defence.
Britain currently has about 8,000 military personnel in four southern Iraqi provinces, mostly based around the city of Basra.
Defence Secretary John Reid announced on January 20 that about 230 British troops had been injured in enemy action since the war in Iraq began.
Of those, 40 suffered life-threatening injuries.