Three soldiers from the Highland Fusiliers 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 yesterday.

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 not only look at how the bomb was made, but also into 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.

"I've not seen a technical report but my understanding in talking to the Ministry of Defence is that in all probability it was a very large device in terms of the amount of explosive and it may well have physically lifted up the vehicle and possibly even turned it over."

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.

Scotland's First Minister Alex Salmond today said: "This is tragic news and our thoughts and deepest sympathies are with the families and loved ones of the soldiers who have been lost.

"This incident demonstrates once again the dangers faced by our armed forces, often on a daily basis, and they deserve our deepest gratitude and respect for the job they do in some of the most difficult and trying circumstances imaginable.

"No words can truly bring comfort to the families at a time like this, but the thoughts and prayers of everyone in Scotland will be with them upon hearing this dreadful news."

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".

A Ministry of Defence spokesman said that yesterday's incident underlined the continued danger faced by soldiers as they prepare to pull out of Afghanistan in 2014.

He said: "Security in Helmand, where most UK forces are based, is steadily improving with Afghan forces already responsible for the bulk of the province - but the environment in which our troops operate remains risky and dangerous, including the threat of improvised explosive devices and insurgent attack.

"We will continue to do all we can to minimise these risks but they can never be removed entirely."

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.

The attack came on the third day of what the Taliban has called its spring offensive.

This year's "fighting season" is thought to be a key test, as the international coalition prepares to hand over security responsibilities to Afghan forces next year.

Around 350 soldiers from the Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland (2 SCOTS) were deployed to Afghanistan in March for a six-month tour.

The battalion, based at Glencorse Barracks in Penicuik, Midlothian, is carrying out a number of roles including mentoring and training Afghan National Security Forces, advising Afghan police chiefs and helping with the training of new officers.