By Caroline Wyatt
Defence correspondent, BBC News
The Viking will be replaced by a new vehicle, the Warthog, next year
The most recent British deaths in Afghanistan have prompted fresh debate over the Viking armoured vehicles used by British forces in Helmand province.
The Ministry of Defence, as well as serving and former army officers, have defended the use of the Viking, while others say that as the Taliban plant ever bigger improvised explosive devices, it is no longer suitable for this particular battlefield.
The vehicle was deployed to Afghanistan three years ago with the Royal Marines, who were already using it as an amphibious craft.
The Viking proved extremely useful as a tracked vehicle capable of negotiating the ditches, canals and uneven ground of the Green Zone around the Helmand River.
It enabled British forces to go off-road, helping them to minimise at least some of the risk of hitting roadside bombs by using unpredictable routes.
However, as the threat of improvised explosive devices (IEDs) increased - both in size and sophistication - the Viking was gradually given more protective armour in response.
Lt Col Thorneloe and Trooper Hammond were killed in a Viking
Amyas Godfrey, a former infantry officer and associate fellow of the Royal United Services Institute think tank, says that the choice of which armoured vehicle to use in any campaign is a question of balancing risks and benefits.
"It is all about getting the balance right between the need for armour and the need to be light and flexible, with the ability to go off-road," he told the BBC.
"Mobility is a form of protection in itself, and with heavier armour, you sacrifice mobility for greater protection."
Even Mastiffs and other more heavily-armoured vehicles used by British forces in Helmand have proved vulnerable to the Taliban's biggest IEDs and mines.
And defence sources say that the size of the bomb which hit Lt Col Rupert Thorneloe and Trooper Joshua Hammond's Viking would have caused significant damage to any armoured vehicle.
The spokesman for Task Force Helmand, Lt Col Nick Richardson, insists the Viking remains an excellent vehicle.
"It has superb mobility that allows it to go almost anywhere and therefore minimise the risk of hitting an IED," he said.
"Armour is the last resort in terms of defeating the threat. It is much better to be able to avoid the threat than to have to rely on the armour defeating the threat when it is initiated.
"It doesn't matter how much armour you have - it can always be overcome if you make the charge big enough."
Retired army officer and author of the books An Ordinary Soldier and the forthcoming Task Force Helmand, Doug Beattie MC was serving in Afghanistan when the Viking was first brought in.
"They had extra armour to protect them from RPG [rocket-propelled grenade] attack on the flanks, but the biggest problem was the vehicle's low profile to the ground," he said.
"It was so close to the ground that the base of the Viking was very vulnerable to improvised explosive devices.
"The MoD was not negligent in using them, but the threat changed over time as the battlefield developed. It was a useful vehicle for what it was, but not something I would like to use in Afghanistan now."
Raised threat level
But he points out that it takes time and trials to bring in a new vehicle, and every time new protection for British forces is introduced in Helmand, the Taliban adapt their tactics accordingly.
The MoD admitted in 2008 that the Viking had reached the limit of how much more it could be armoured.
It is due to be replaced by the new armoured, all-terrain Warthog vehicle in Afghanistan next year, while a new class of mine-clearing vehicles, to include the Buffalo, is also being developed.
On his recent visit to Helmand province for Armed Forces Day, the Defence Secretary Bob Ainsworth told the BBC that the MoD was responding to the raised level of threat from roadside bombs.
"We are getting new expertise and new equipment into theatre, and we're doing that as quickly as we can in order to mitigate that risk," he said.