Three British soldiers have been killed and six others injured in a huge roadside bombing in Afghanistan - the first troops to have died in a heavy-armoured Mastiff vehicle.

The soldiers, Corporal William Savage and Fusilier Samuel Flint of the 2nd Battalion, Royal Regiment of Scotland, and Private Robert Hetherington, of the 7th Battalion, Royal Regiment of Scotland, were travelling in the Nahr-e Saraj district of Helmand on a stretch of road that had been in regular use and was believed to be in a safe area.

Their deaths come as international forces prepare to leave the 12-year-old war.

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Military officials are investigating whether insurgents are designing bigger bombs aimed at piercing the 23-tonne Mastiff's armour. They will not only look at how the bomb was made, but also into checks used by Afghan and 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.

The convoy was on Route 611, a new road that had been used to show the reconstruction taking place in the province. Increasingly, the Afghan army and police are carrying out searches for IEDs as they take over security duties.

The last time so many British soldiers were killed in one incident was last March when six died after their Warrior vehicle was blown up by an IED about 25 miles north of Bashkir Gash.

The Taliban announced last weekend its “spring offensive” had started with a series of bombings. The insurgents said they were running large parts of the country, publicising the case of a father who executed his 18-year-old daughter, on their orders, after she had run away from home.

Former Army chief of staff Lord Dennett told the BBC: “As in any cycle of conflict, there's invention and counter-invention. The Taliban have found a way of countering the protective qualities of the Mastiff.”

Cpl Savage's pregnant wife, Lindsey, said: “I have lost the love of my life and the father of our son.” The 21-year-old joined the Army in April 2003 and had previously completed tours of both Iraq and Afghanistan.

Lieutenant Colonel Robin Lindsay, Pte Hetherington's commanding officer, said the 25-year-old had epitomised “everything that is excellent about the reserve forces”. The US-born soldier had enlisted in the Territorial Army in 2006 and hoped to attend Royal Military Academy at Sandhurst.

The Prime Minister David Cameron said that 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”.

Scotland's First Minister Alex Salmond said: “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.”

The deaths bring the total of UK military losses since 2001 to 444, at a time when UK forces have drastically scaled down operations with Afghans taking the lead. Commanders have become concerned that this has created a false sense of security for the public back home, especially with the onset of the fighting season.