In a statement, the ministry says a bomb blast killed three Royal marines in the Sangin area of Helmand province on Friday, while another Royal marine died in a separate blast south of Sangin.
British Prime Minister Gordon Brown released a statement calling the deaths a "tragic loss" and saying fighting the Taliban in Afghanistan puts British forces in the front line against terrorism.
Elsewhere in southern Afghanistan, NATO said its troops fired on a speeding bus that failed to heed warning shots on Friday, killing at least three civilians on board.
The alliance says a NATO foot patrol fired warning shots as the bus veered toward the troops on the highway that links Kabul to Kandahar.
Civilian deaths at the hands of international troops have caused tension between the Afghan government and western governments.
Afghan President Hamid Karzai has repeatedly called on international forces to do all they can to prevent civilian casualties during military operations.
The U.S. Navy issued a statement Friday, saying President Karzai visited a U.S. aircraft carrier, USS Theodore Roosevelt, the day before on Thursday to gain a better understanding of how the U.S. military carries out air operations against suspected insurgents.
In other news, a U.S.-based rights group is calling on the United States and Afghanistan to investigate reports that a mass grave in northern Afghanistan has been tampered with.
Physicians for Human Rights released a statement on Friday saying an Afghan warlord, General Abdul Rashid Dostum, removed evidence of potential war crimes at the Dasht-e-Leili mass grave. The group says the U.S.-allied Afghan commander killed hundreds of prisoners and dumped their bodies at the site seven years ago.
Some information for this report was provided by AFP, AP and Reuters.