Three British soldiers were killed and six others injured after their heavily-armoured vehicle was hit by a large roadside bomb.
The servicemen, from The Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland, died while on a routine patrol in the Nahr-e Saraj district of Helmand Province on Tuesday.
The deaths mark the first time British troops have died in an armoured Mastiff vehicle since it was introduced to the campaign in 2007. The three soldiers were airlifted to the military hospital at Camp Bastion but could not be saved. It is unclear how badly injured the other six soldiers were.
It is understood the explosion was caused by a particularly large bomb and officials are looking into whether insurgents are designing bigger bombs aimed at piercing the Mastiff's heavy armour. It is unclear whether the bomb penetrated the 23-tonne vehicle or lifted it off the road and turned it over, but it is the first time troops inside a Mastiff have been killed by an IED (improvised explosive device) attack.
An investigation is expected to look at how the bomb was made, and how the incident happened, including techniques and checks used by British troops on patrol.
"If you build a big enough bomb it will overcome a Mastiff. It's just never been done before," a military source said, adding that the answer would not necessarily be to add more armour to the vehicle, or create different armoured vehicles, but to continue to work to prevent attacks.
Former Army chief of staff Lord Dannatt told Radio 4's Today programme: "One has to accept tragically that, as in any cycle of conflict, there's invention and counter-invention. The Taliban have found a way of countering the protective qualities and characteristics of the Mastiff. It would seem that this was an extremely large bomb that was so powerful that actually it was able to cause fatalities within the vehicle itself."
The last time so many British soldiers were killed in one incident was last March when six died after their Warrior armoured vehicle was blown up by a massive IED about 25 miles north of Lashkar Gah.
Prime Minister David Cameron said the country had paid a "very high price" for its work in Afghanistan, but maintained it was "vital" in making sure the country "doesn't again become a haven for terrorists".
The deaths take to 444 the number of UK service members who have lost their lives since operations in Afghanistan began in October 2001. Six have died in 2013.