British forces in Afghanistan have used one of the world’s most deadly and controversial missiles to fight the Taliban.

Apache attack helicopters have fired the thermobaric weapons against fighters in buildings and caves, to create a pressure wave which sucks the air out of victims, shreds their internal organs and crushes their bodies.

The Ministry of Defence (MoD) has admitted to the use of the weapons, condemned by human rights groups as “brutal”, on several occasions, including against a cave complex.

The use of the Hellfire AGM-114N weapons has been deemed so successful they will now be fired from RAF Reaper unmanned drones controlled by “pilots” at Creech air force base in Nevada, an MoD spokesman added.

Thermobaric weapons, or vacuum bombs, were first combat-tested by the Soviet Union in Afghanistan in the 1980s and their use by Russia against civilians in Chechnya in the 1990s was condemned worldwide.

The secret decision to buy the Hellfire AGM-114N missiles was made earlier this year following problems attacking Taliban fortified positions.

British Apache pilots complained that standard Hellfire antitank missiles were going straight through buildings and out of the other side. Even when they did explode, there were limited casualties among the Taliban inside, particularly when a building contained a number of rooms.

American Apache pilots overcame the problem in Iraq with the thermobaric Hellfire.

The weapons are so controversial that MoD weapons and legal experts spent 18 months debating whether British troops could use them without breaking international law.

Eventually, they decided to get round the ethical problems by redefining the weapons.

“We no longer accept the term thermobaric [for the AGM-114N] as there is no internationally agreed definition,” said an MoD spokesman. “We call it an enhanced blast weapon.”

The redefinition has allowed British forces to use the weapons legally, but is undermined by the publicity of their manufacturer, Lockheed Martin, which markets them as thermobaric.

When the American military bought them in 2005, President George W Bush said: “There are going to be some awfully surprised terrorists when the thermobaric Hellfire comes knocking.”

Despite the Bush rhetoric, it is unlikely anyone targeted by the missile would know much about it. The laser-guided missile has a warhead packed with fluorinated aluminium powder surrounding a small charge.

When it hits the target, the charge disperses the aluminium powder throughout the target building. The cloud then ignites, causing a massive secondary blast that tears throughout any enclosed space.

The blast creates a vacuum which draws air and debris back in, creating pressure of up to 430lb per sq in. The more heavily the building is protected, the more concentrated the blast.

The cloud of burning aluminium powder means victims often die from asphyxiation before the pressure shreds their organs.

Jim Gribschaw, Lockheed Martin’s programme director for air-to-ground missiles systems, said the thermobaric Hellfire was “capable of reaching around corners to strike enemy forces hiding in cases, bunkers and hardened multi-room complexes.”

Human Rights Watch argues they are “particularly brutal” and that their blast “makes it virtually impossible for civilians to take shelter”.

Nick Harvey, the Liberal Democrat defence spokesman, said: “It is staggering the MoD has added these weapons to Britain’s arsenal in cloak-and-dagger secrecy. Parliament has never assented to their use.”

He added: “Gordon Brown claimed the moral high ground when Britain supported a ban on cluster munitions but leaving a loophole for these weapons casts a different picture on the true position.”

The MoD said: “We are conscious of the controversial aspects [of this weapon] but it is being used sparingly and under strict circumstances where it is deemed appropriate by the commander on the ground.”

A spokesman added that it could “achieve objectives with the minimum coalition casualties and reduced collateral damage”.

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