Almost one in five of Britain's elite SAS troopers have become a casualty in a secret war against hardline al Qaeda fighters in Iraq, according to military sources.

While the Hereford-based 22 Special Air Service Regiment is credited with killing or capturing up to 3500 insurgents in Baghdad and the provinces around the capital, the campaign has cost the unit seven dead and more than 50 wounded.

The SAS numbers fewer than 300 "shooters" organised in four "sabre squadrons". The casualties include 30 with critical wounds which have resulted in amputation of one or more limbs or life-threatening head injuries.

The loss rate is more than twice the 10% average even for conventional infantry units which have taken part in the heaviest fighting in Afghanistan in the last two years.

It is also the heaviest loss rate of special forces' members since the crash of a helicopter in the South Atlantic in 1982 killed 18 experienced soldiers, including a number of NCOs.

The losses have a greater impact because of the exp-erience and training needed to join the SAS and a growing shortage of qualified replacements. All of the dead and wounded were sustained long after the initial invasion in 2003 when SAS squadrons were rotated through a covert intelligence gathering and counter-insurgency group known as Task Force 88.

The group, later renamed Task Force Black, also includes Delta Force, the US equivalent of the SAS, which has also sustained at least 20% casualties in the same undercover operations.

The "black ops" combined unit operates from a building in Baghdad's Green Zone known as the Station House, but has an outstation at Balad, north of the Iraqi capital in the Sunni insurgent heartland.

It was SAS troopers who located Saddam Hussein's two sons, Uday and Qusay, in the city of Mosul in 2003.

Both were killed when US commanders insisted on taking charge of such a high-profile capture operation.

They deployed missile-firing helicopters and 200 soldiers. Four US personnel were wounded.

The SAS had a 12-man snatch team on site and are still convinced they could have taken the targets alive.

The combined special forces are now credited with removing some of the most dangerous terrorists in Iraq from the streets of the capital and with breaking the back of al Qaeda in Iraq, who have been operating a highly effective car-bomb campaign in the country.

Suicide bombing attacks in Baghdad have declined from 150 a month 18 months ago to two a month this year.

General David Petraeus, the head of the US forces in Iraq who is due to leave his post shortly, has praised the courage of the SAS.

He said: "They have helped immensely in Baghdad. They have done a phenomenal job."

In one incident, SAS troops rented a pink pick-up truck before they removed their body armour to blend in with locals and drove through the traffic to capture a key high-value target.

"It was brilliant, actually," said Gen Petraeus.

"They have exceptional initiative, exceptional skill, exceptional courage and, I think, exceptional savvy.

"I can't say enough about how impressive they are in thinking on their feet."

SAS snipers are also credited with picking off a series of bombers equipped with explosive suicide vests before they could detonate their bombs.