The fatalities bring the British death toll in Afghanistan since the 2001 U.S.-led invasion up to 91.
The ministry said in a statement: "Just after 1653 hours local time, the Marines were conducting a patrol in the vicinity of Kajaki, Helmand province, when the vehicle they were travelling in was caught in an explosion."
"Medical treatment was provided prior to both being evacuated to the field hospital at Camp Bastion. Despite the best efforts of the medical team, both sadly died as a result of their wounds."
Afghanistan's Defense Ministry said on Monday that 12 Taliban fighters were killed and several captured over the weekend in the country's south.
A source in the ministry told RIA Novosti that five Dutch soldiers were injured in Tarin Kowt, in the Uruzgan province on Sunday, when roadside bombs hit two armored vehicles they were driving in, contradicting an earlier report from the Dutch Ministry of Defense that three soldiers were wounded.
Taliban fighters have pledged to step up attacks against Afghan and foreign troops this year, in their drive to topple the country's pro-Western government.
The Taliban carried out over 100 suicide bomb attacks, killed 200 foreign troops and several thousand civilians in 2007. Over 30 foreign soldiers have been killed in Afghanistan this year.
The Taliban continues to receive huge revenue from the country's opium industry, which national officials say accounts for 93% of global production, with annual turnover of around $4 billion.
