After six British soldiers are presumed killed in an explosion in Afghanistan, pushing the death toll above the 400 mark, Channel 4 News asks how so many could have died in a newly-reinforced vehicle?

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The six soldiers are all listed missing, presumed dead, after their Warrior armoured fighting vehicle was hit by an explosion in southern Afghanistan.

The incident is the biggest single loss of life for British troops engaged in the conflict since 2006. Last June the Ministry of Defence (MoD) carried out an upgrade of 70 Warrior vehicles in use in Afghanistan.

Five of the dead soldiers were from the 3rd Battalion, the Yorkshire Regiment, and one from the 1st Battalion, the Duke of Lancaster's Regiment. They were on a mounted patrol in Helmand province last night when their Warrior was hit by an explosion, according to the MoD. If their deaths are confirmed the number of British soldiers killed since the start of operations in Afghanistan will stand at 404.

Afghanistan casualties: click here for an interactive timeline of British troop deaths

As yet, full details of the nature of the blast and the extent of the damage remain unclear as investigators comb through the wreckage to try and recover the little that is left.

Defence Secretary Philip Hammond told Channel 4 News: "As tragic as this incident is, the overall trend of successful attacks against British forces is down, casualties among British forces are down and we know that the Taliban insurgency is suffering significant difficulties - logistical difficulties, difficulties recruiting, morale difficulties - so I wouldn't agree that the mission is not succeeding.

"We don't know the full details of this particular incident yesterday, why this very heavily armoured Warrior vehicle was so badly affected by this particular IED. It may have been simply a fluke occurrence."

While some have asked why the soldiers, rather than Afghan security, were patrolling that area in the first place, other issues surround levels of military intelligence, given the Taliban were able to plant such a deadly bomb under complete cover.

Soviet landmine?

It is understood that the blast was not thought to have come from a rocket propelled grenade but from something closer to a roadside bomb. An MoD source told Channel 4 News: "The damage was caused to the underneath of the vehicle. By the type of damage that has been done, it is likely that it was an IED [improvised explosive device] or an older Soviet landmine."

Some reports have suggested that the blast was so large it triggered a blaze, setting off the ammunition and explosives on board the vehicle. It is reported that the secondary explosions which followed as a result of the ammunition being set off prevented soldiers in the second Warrior to get to their colleagues and mount a rescue.

Upgrade

Already regarded the British Army's second toughest vehicle in Afghanistan after specialist mine protected vehicles, 70 Warrior Infantry Fighting Vehicles underwent a £40m refit of protective material last June. The work, carried out under MoD contract by weapons manufacturer BAE Systems, included installation of a modular armour system "to allow quick and easy fitment of 'mix and match' armour packages to meet changing threats", according to the company.

"By the type of damage that has been done, it is likely that it was an IED or an older Soviet landmine." MoD Source

BAE Systems also fitted enhanced seating and armour to further improve mine protection, and raised suspension and ride height for safety and to restore cross-country mobility.

Further additions specifically designed for fighting in Afghanistan were also included, believed to be a large belly plate under the vehicle for further protection.

"The trade-off," said a source close to the contract, "is between mobility and protection. Once you add to the armour, you make it [the vehicle] less mobile, and more predictable and more easy to attack. All the extra weight made it [the Warrior] grow by 25 per cent, so it sank down on its springs. The suspension was beefed up to restore the ride height, and improve mobility."

But despite additions, modifications, enhancement and improvements, "if a blast is big enough, there comes a point where there is nothing you can do," said the defence source.

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Deficiencies

Previously, when such vehicles have been targeted by blasts, they have been swept into the air and landed by the side of the road. This is the first time that a blast has caused such extensive loss in one go.

Many of the modifications were in response to obvious deficiencies, as in 2008, when an inquest revealed that four soldiers were killed in Iraq inside a Warrior after a bomb ripped through the unprotected underside. During the inquest, it emerged the vehicle was armour-protected on top and sides, but not on the bottom. Another British soldier, L/Cpl Ryan Francis, also died in an explosion in Basra after sensitive bomb detection equipment was switched off.

Yet if weight of the bomb which tore through the underside of the Infantry Fighting Vehicle could be as much as several hundred kilos, it will be near-impossible to manufacture a vehicle with complete protection and with adequate mobility.

"At the top of the threat profile, we're not dealing with a load of muppets." Francis Tusa, Defence Analysis

Crucially, however, defence analysts say there is a general failure among those not involved in the conflict to understand the second-guessing and game playing involved when two sides are engaged in a war, and a failure to appreciate the ever changing tactics of a sophisticated Taliban insurgency.

Francis Tusa, editor of Defence Analysis, told Channel 4 News: "At the top of the threat profile, we're not dealing with a load of muppets. We are dealing with a very, very sophisticated set of people. We shouldn't play down the adaptiveness and the cleverness of some of the Taliban units - we shouldn't underestimate them.

"There is a constant process of evaluating technology, on both sides. There is new kit coming into service. The Taliban were using IEDs which had no impact at all on vehicles, so they then started changing how they placed IEDs, and they doubled the explosive size of the IEDs, so over the last five years, there has been a drive towards more poweful IEDs."

Here, said Mr Tusa, lies the other problem. "In Afghanistan, there is no shortage of explosives," he said. "With the Russian war previously, almost on a daily basis, troops engaged in normal procedures come across weapons caches. The country is awash with explosives.

"But undoubtedly, there will be questions asked. Spending [to protect troops] is millions a year, and a politician would have to be particularly brave to cut budgets for force protection."