Prime Minister David Cameron said it was "a desperately sad day for our country" as news emerged of the attack in Helmand.
Speaking in Downing Street, he said: "This is a desperately sad day for our country and desperately sad of course for the families concerned.
"It is a reminder of the huge price that we are paying for the work we are doing in Afghanistan and the sacrifice that our troops have made and continue to make.
"I do believe it's important work for our national security right here at home but of course this work will increasingly be carried out by Afghan soldiers and we all want to see that transition take place.
"But today we should think of the families."