The growing number of coalition troops killed by their Afghan allies is raising questions about the recruitment procedures of Afghan security forces - and the trustworthiness of the tribal leaders who endorse each recruit.

"There were two officers who came up and rang the doorbell. Like they say in the movies, you don't want to see the suits come to the door," recalls Rod Watkins.

"As soon as they asked if I was Paul's father, I knew something was wrong."

Rod's son Paul, a Lance Corporal with the Royal Lancers, had been killed in Afghanistan. He was 24-years-old.

Image caption L/Cpl Paul Watkins was killed by a soldier of the Afghan National Army while on patrol in Helmand.

From then on, it only got worse. Rod soon discovered that Paul had not died in combat. He was shot by a soldier of the Afghan National Army - a man he was helping to train and who was supposed to be his ally against the Taliban.

Mr Watkins was not satisfied with the account he received from the military of exactly what happened, so from his home in Port Elizabeth, South Africa, he pieced the story together using evidence from the inquest and what he gleaned from his son's comrades.

People must understand that this is a Taliban tactic and it's potentially a very successful tactic because it reduces trust and lowers morale Lord Richard Dannatt, Former Chief of the General Staff

Paul was part of a unit which had been sent out short-handed. There were only seven men instead of eight - the missing man was on R&R (Rest and Recuperation).

So, as the other six went out on foot patrol in dangerous territory in Helmand province, Paul was left to guard both their vehicles with two Afghan soldiers.

One of them put his rifle down and went to relieve himself in the bushes. The other saw his chance and fired two bursts of automatic fire straight at Paul. He hit him six times.

"He had two shots to the brain - he would have died before he hit the ground," says Mr Watkins.

"I am very upset with the British Army because I entrusted my son to them and they didn't look after him properly. That's my personal feeling," he told Radio 4's File on 4.

"Something went wrong and they say sorry afterwards, but sorry doesn't bring back my son."

51 deaths this year

Military commanders call these rogue attacks "green-on-blue" - green for Afghan forces and blue for the coalition. They have accounted for 114 deaths since 2007 - which includes 18 British soldiers.

Green on Blue: The Numbers 114 members of the coalition forces have been killed by members of the Afghan services since 2007.

This includes 18 British soldiers who have been killed by Afghan recruits since 2008.

51 members of the coalition forces have been killed by Afghan forces in 2012 - 9 of these were British.

1 in 7 coalition casualties are now the result of green on blue attacks.

In 2004 the Afghan National Army numbered 24,000 - today it has around 195,000 troops.

In 2004 the Afghan National Police numbered 33,000 - today it has around 150,000 officers. Source: NATO Read more: UK military deaths in Afghanistan

The official NATO analysis says about a quarter of rogue attacks are the result of Taliban infiltration or influence. The rest are due to individual grudges or disputes which get out of hand.

However, the rate of attacks is increasing and this year has been by far the worst, with 51 coalition casualties - 9 of which were British. One in seven coalition casualties is now the result of a green-on-blue killing.

The former head of the British Army General Lord Dannatt sees this as a deliberate policy adopted by enemy fighters:

"People must understand that this is a Taliban tactic and it's potentially a very successful tactic because it potentially reduces trust and lowers morale among troops engaged in the operation.

"And of course it provokes a reaction at home: if we are supposed to be helping them why are they killing us?"

At the end of July this year, Richard Chamberlain was lying in bed, reading, at his home in Broadstairs in Kent when his mother started knocking frantically downstairs. Again there was an awful sense of foreboding.

"When there's a knock at the door at 10:30pm, it's rarely good news," says Richard.

Richard's brother David had also been killed in Afghanistan - although he was not in the army, but an experienced senior customs official working under contract as a civilian mentor. He was killed while training officers of the Afghan customs and border police in Herat.

"Dave enjoyed the work and the camaraderie. He believed that they were there for a good reason, even a noble reason," says his brother.

The facts have yet to emerge in detail but it is known that the man who shot him also killed two others and wounded five more. He was wearing an Afghan police uniform.

Richard sees his brother's death as part of a pattern of treachery in Afghanistan.

"It is happening more and more often now. I suppose they are taking recruits where they can find them.

"It is a near-impossible task to vet Afghans and say these people will not commit an atrocity."

Tribal recommendations

Green on blue attacks raise serious questions about the vetting processes for the Afghan National Security Forces (ANSF).

Conflicting interests Captain Doug Beattie has completed 3 tours of Afghanistan, most recently with the Territorial Army. He told File on 4 about the tribal allegiances which conflict with the work of many Afghan recruits. "Do not think of the Afghan Police as your local policeman who really looks after criminality. What you're looking at is a man who is normally illiterate, who is heavily armed, but has no concept of the rule of law. This young man is policing the area he lives in so they have family, friends and tribal leaders coming up to them and asking them to turn a blind-eye when they are moving a poppy crop through a checkpoint - that happens quite a bit." "But sometimes there is an insurgent who could be known to the policeman who will ask him to turn a blind-eye so he can carry out whatever he intends to carry out. We know this and we've monitored this in some occasions. It's not because the policemen is aligned to the insurgency, it's not because he is a Taliban who has joined the police, it is because of these external influences against him from his family, from his tribe."

How do they let in recruits who are either Taliban loyalists in disguise or so hot-headed that they respond to a grievance with murder?

The NATO Parliamentary Assembly will meet in November to review progress in Afghanistan and will consider a draft report from its Defence and Security Committee which says that tighter vetting procedures were introduced in February of this year - five years after green on blue attacks began.

The author Sven Mikser, of the Estonian parliament, acknowledges that in a poor and largely illiterate country like Afghanistan there are bound to be weaknesses in the system.

"It's a country with more than 30 years of war, so when it comes to nationwide databases and registers then they have been non-existent," he tells File On 4.

"There are biometric databases and other databases, which in the past have been inadequate but are getting better and better."

A large part of the vetting process depends on a distinctly Afghan form of job reference - the recommendation of a tribal elder. Mr Mikser says this is helpful and an important part of procedure - but he accepts its limitations.

"It requires the local elders to also be fully trustworthy sources of information... our service people in Afghanistan are in a good position to assess, but obviously in a country like Afghanistan nothing can be 100% fool-proof."

Something the families of David Chamberlain and L/Cpl Paul Watkins know too well. The men who killed them were both recruited into the ANSF via tribal leaders and both were on duty at the time they attacked.

At the coalition HQ in Kabul, spokesman Brigadier General Gunter Katz defends the reliance on tribal elders in the recruiting process.

"It is important to note that this vetting process is an Afghan-led process and they know the best about their culture and about their honour and pride in this regard.

"So when our Afghan partners rely on the elders in those villages we believe they know exactly what they are doing and trust them."

He points to hundreds of members of the Afghan forces who have recently been dismissed following re-vetting.

"Maybe the first vetting process was not solid enough. Maybe there was some new information.

"It is a case-by-case issue here and I think the Afghans take it very seriously. The fact that they do the re-vetting again - very successfully, I have to say - shows how important it is for them."

When NATO decided this month to restrict future joint operations with Afghan forces, it indicated that its motive was to preserve the safety of its own troops.

But Brigadier General Katz says the coalition remains committed to partnering, training and assisting the joint operation - something that is essential, he argues, if the Afghan army and police are to be trained to take control of their own country.

That is the key to a policy which is seen as vital in Britain, the United States and throughout NATO - withdrawal of their troops by the end of 2014.

Listen to the full report on File on 4 on BBC Radio 4 on Tuesday, 24 September at 20:00 BST and Sunday, 30 September at 17:00 BST. You can also listen again via the Radio 4 website or File on 4 download.