Two British soldiers were killed when a man wearing an Afghan police uniform opened fire on them in Helmand province, southern Afghanistan.

The soldiers, from 1st Battalion The Royal Gurkha Rifles, were shot dead at a checkpoint in Nahr-e Saraj on Tuesday, the Ministry of Defence said. Their families had been informed.

Major Laurence Roche, spokesman for taskforce Helmand, said: "The loss of these soldiers is a huge blow to The Royal Gurkha Rifles and everyone serving in taskforce Helmand. Our thoughts are with their families, friends and fellow Gurkhas at this time."

An investigation has been launched into what appears to have been the latest insider or "green-on-blue" attack on coalition troops by members of Afghan security forces, a trend that Nato's secretary general has admitted is sapping trust and has already forced the US military to limit joint operations with Afghan troops.

The surge in attacks has also led to doubts that Afghan forces will be ready for the planned takeover from international troops in 2014.

The latest attack reportedly happened in or near Gereshk, a town at the heart of Helmand's central district of Nahr-e Saraj, the most violent part of the province.

The killing of the Gurkhas would bring the UK death toll from green-on-blue attacks to 11 so far this year. There was only one such British death last year, three in 2010 and five in 2009. In all, 437 UK services personnel or civilians working for the MoD have now died in Afghanistan since October 2001, 394 of them killed in action or dying of their wounds.

More than 50 international troops have been killed by Afghan soldiers or police in 2012. More than 50 Afghan members of the government's security forces have died also in attacks by their own colleagues.

A spokeswoman for the Nato-led International Security Assistance Force said: "An individual wearing the uniform of an Afghan policeman turned his weapon against two Isaf service members in southern Afghanistan, killing both."

Abdul Nabi Elham, provincial police chief for Helmand, said: "It happened before noon, I don't know exactly when, in the Gereshk area.

"I checked with ANP [Afghan national police] and ANA [Afghan national army], there are no soldiers who opened fire on foreigners. It seems the guy who opened fire should be a Talib. We have sent a delegation to the site to do a full investigation."

A Taliban-linked Twitter account @Abalkhi said: "Afghan police officer (Atiqullah) opens fire on foreign troops, 12pm local time … hero grabs enemy machine gun, joins Mujahideen nearby."

The latest British deaths come less than a week after army medic Corporal Channing Day and Royal Marine Corporal David O'Connor were killed. Day, who served with 3 Medical Regiment, and O'Connor, of 40 Commando, were injured on patrol with C Company in Nahr-e Saraj.

An initial review into Day and O'Connor's deaths has revealed they were not caused by friendly fire. The MoD said an Afghan man, believed to have been a member of the Afghan uniformed police but who was not wearing uniform at the time, also died during the incident, leading to speculation the incident was a green-on-blue attack.

The strains caused by such attacks on Nato's partnership with Afghan authorities are growing, with coalition leaders being increasingly outspoken on the issue.

Philip Hammond, the defence secretary, told the Guardian in September that while Britain would not be spooked by green-on-blue attacks, "public opinion feels very strongly" and "it is absolutely essential that this problem is put back in its box". But he insisted then that the Afghans were moving to address the problems and that "we are all over it".

Earlier this month Nato's secretary general, Anders Fogh Rasmussen, said: "There's no doubt insider attacks have undermined trust and confidence, absolutely."