Afghan military commanders requested assistance in Sangin district, an area British forces defended from the Taliban until 2010, earlier this month.

About 80 members of 4th Battalion The Rifles, based at Camp Bastion in Helmand province, were involved.

The Ministry of Defence said some insurgents were killed or captured but there were no British casualties.

According to the Sunday Times, Defence Secretary Philip Hammond had personally authorised the return to the area for British forces.

It said 106 British personnel were killed in fighting there between 2006 and 2010.

The paper said the threat to the troops was considered so great that, at the MoD's request, it delayed reporting the operation until it had finished.

The British involvement was part of a major operation by 215 Corps of the Afghan National Army (ANA) to clear insurgents in the Sangin district of Helmand Province, in the south.

The operation saw troops from the UK-mentored 3/215 Brigade move north into Sangin, clearing compounds and seizing insurgent weapon stashes alongside soldiers from 2/215 Brigade.

The Brigade Advisory Group, made up of 4th Battalion The Rifles, provided support to 3/215 Brigade.

During the operation, more than 30 improvised explosive devices were found and destroyed by the ANA, and two insurgent vehicles were seized along with ammunition and weapons.

The Ministry of Defence said UK personnel occasionally operated outside of the usual British area of operations in central Helmand in an advisory capacity.

"These out-of-area operations have been a long-standing element of the UK mission in Afghanistan and are completely in line with our current role of providing training, advice and assistance to the Afghan National Security Forces," a spokesman said.

"Between 2006 and 2010, UK forces provided vital security for the population of Sangin, disrupting the insurgency in an area the Taliban had considered its heartland, preventing the spread of violence elsewhere, upholding the authority of the Afghan government in the area and enabling economic development to take place.

"Much was achieved then and has been since. It remains a challenging area and it is now for the Afghan forces to deal with the residual insurgency."

Brig Rupert Jones, Commander Task Force Helmand, said the operation had demonstrated further how effective 3/215 Brigade of the ANA had become.

"Operating in Sangin over the past week, they have moved to another level of performance and independence," he said.

"It has been a very impressive demonstration of what the Afghan National Army can be capable of."