Western operations with Afghan soldiers and policemen are to be scaled back after a string of "green on blue" attacks in which local recruits turn their weapons on international allies.
The move comes amid mounting concern over "green on blue" attacks on international forces. So far this year, 51 international troops have been killed by Afghan forces or militants wearing their uniforms.
Joint patrols and advisory work with Afghan troops will now only be conducted at the battalion level and above. Battalions are much larger military unit consisting of around 300–1,200 soldiers.
Co-operation with smaller units will have to be "evaluated on a case-by-case basis and approved by RC (regional) commanders", the International Security Assistance Force (ISAF) said in a statement.
The Ministry of Defence said there had been 18 "green on blue" UK fatalities since 2008, including the nine killed this year. In 2011, there was only one such death.
Afghan National Security forces wear green, while their policemen wear blue
At the weekend two British soldiers were killed by an attacker dressed as an Afghan policeman and feigning injury.
Father-of-two Sergeant Gareth Thursby, 29, and Private Thomas Wroe, 18, were shot dead at a checkpoint on Saturday in the south of Nahr-e Saraj district, Helmand province.
ISAF insisted in its statement that it remained committed to its partnership with the Afghan National Security Forces. (ANSF)
Isaf forces with Afghan policemen (in blue)
"Most partnering and advising will now be at the Kandak (Battalion) level and above," the statement said.
"This does not mean there will be no partnering below that level - the need for that will be evaluated on a case-by-case basis and approved by RC (regional) commanders.
"In some cases, ANSF are fully capable of increased independent activity and their advisers will simply be stepping back to advise from the next level.
"Make no mistake about it: we remain committed to our partnership with the ANSF, and we continue to move forward and will meet our campaign objectives."
The ISAF announcement was made as it was revealed that an Afghan and coalition security force has arrested one of the Taliban leaders thought to have been behind the attack on Friday at Camp Bastion, in Helmand province, where the bulk of UK's 9,500-strong force in Afghanistan are deployed..
ISAF said the man was taken into custody by the security force in the Nad 'Ali district by the force following joint efforts by Afghan and coalition forces to track down insurgents responsible for the assault.
The Taliban leader is suspected of having provided support for the attack in which two US Marines were killed and six planes destroyed.
Taliban sources claimed Bastion was targeted because Prince Harry is serving there as an Apache attack helicopter gunner.
Prince Harry was moved to a guarded location during a recent Taliban attack on Camp Bastion, the Defence Secretary has revealed
Mr Hammond had earlier told the Commons that Captain Wales - as Harry is known to the Army - was never in danger although he was present during the assault on Bastion last Friday.
He later explained that the 28-year-old is being protected by "additional security arrangements" as he carries out his second tour of duty in Afghanistan.
Speaking on BBC 2's Newsnight last night, he said: "Clearly there are fall-back plans and I can't go into the detail of them - but once we knew on Friday night that the perimeter at Bastion had been breached he would have been moved to a secure position under effective guard."
Asked by presenter Jeremy Paxman if that meant Prince Harry was not treated the same as every other soldier, the Defence Secretary said: "You asked me whether he was at any greater risk.
"And I've told you that in combat he's at the same risk as any other Apache pilot.
"Clearly if we had a VIP in theatre and frankly if I was there or, Jeremy, if you were there in Camp Bastion and there was a breach of the perimeter security, anybody who might, by nature of who they are, be a target, they would be put in a secure location."
He added: "He is serving there as an ordinary officer but clearly there are additional security arrangements in place that recognise that he could be a target himself specifically as a result of who he is."
The Prince was about two kilometres away with other Apache crew members during the assault on the base, which is the same size as Reading.
Answering an urgent question in the Commons, Mr Hammond insisted the strategy of "mentoring and training" Afghan army and police was vital to the war effort, adding: "We cannot and we will not allow the process to be derailed."
Afghan deputy foreign minister Jawed Ludin said he understood Nato's efforts to protect its forces but insisted the move did not represent a "break in the partnership" to train Afghan forces to take over in 2014.
"This insider attack phenomenon is a matter of deep concern for all of us," he told BBC Radio 4's Today programme.
"This is a challenge that has come up in what is really a very essential mission and that is to get the Afghan forces up to speed.
"We are fully aware and conscious of this and how important it is and that it could affect our joint effort and we are trying our best.
"For Nato to take measures to reduce the risk of these attacks being repeated is a very natural thing. We totally understand it.
"But this should not be overstated because these are just some of the measures to reduce risk; they do not in any way represent a break in the partnership that exists on a day-to-day basis and this will continue."
He said Kabul was "not in any way concerned about the strategic impact" at this stage.
"What we are concerned about is that this could, if not prevented, lead to a strategic setback.
"But at this stage it is just a measure, with other measures that are required.
"There are so many alternatives there. Joint operations are crucial but they could be conducted in a way that reduces the risk of Nato soldiers suffering these attacks."
On Friday, Lance Corporal Duane Groom, from 1st Battalion Grenadier Guards, died in Afghanistan after his vehicle hit a roadside bomb.
The weekend's bleak news brought the total number of members of UK forces to have died since operations in Afghanistan began in October 2001 to 430