Intelligence reports show that rising numbers of home-grown jihadists have joined the Taliban so they can kill British soldiers.
Senior military sources say UK troops are engaged in a 'surreal mini-civil war' in the dusty badlands of Helmand Province.
The revelations came as the Ministry of Defence announced another three British soldiers had been killed in a roadside bomb attack in southern Afghanistan today.
UK troops in action in Helmand are now fighting fellow Britons, it is believedThe soldiers, from 1st Battalion The Rifles, were hit by the blast near Lashkar Gar on Wednesday morning. It brings the British death toll in Afghanistan since the war began in October 2001 to 148.
Surveillance operations from the warzone has picked up voices talking with West Midlands and Yorkshire accents, according to official briefing documents.
The electronic 'chatter' confirms mounting fears that young disaffected Muslims are travelling to southern Afghanistan to commit violence against British targets.
MI5 has estimated that up to 4,000 British Muslims had travelled to Pakistan and Afghanistan for military training, with 'dozens' switching to the front-line.
One senior military source said intelligence officers had been hearing more voices speaking in Pakistani accents such as Punjabi, Urdu and Kashmiri Urdu rather than Pashtu, the official Afghan language.
He said: 'There appears to be more men from other parts of Pakistan fighting with the Taliban than just the Pashtuns who have tribal allegiances with the Afghan Pashtuns.
'It is this group, the Urdu, Punjabi speakers, etc, who fall back into English in, for example, Brummie accents.
'You get the impression that they have been told not to talk in English but sometimes simply can't help it.'
Tory MP Patrick Mercer, the chairman of the Commons anti-terrorism sub-committee which has been examining British Muslim extremism, said: 'We know the problem we have with UK-based jihadists.
'We also know that a number of them have been arrested trying to leave the country.
'With the UK intelligence services at full stretch, it is not surprising some of these jihadists have ended up in Afghanistan.'
The main concern of the security services about British Muslims travelling to Pakistan has been the part they played in terrorist atrocities and plots in the UK.
But after RAF spy planes 'listened in' to the conversations of Taliban fighters, officials now believe the jihadists are mounting missions against British and Western targets in the warzone.
The Ministry of Defence refused to comment on the reports.
A Royal Marine from 42 Commando fires a mortar in Helmand, where members of the unit say the Taliban are now resurgentMeanwhile, the MoD confirmed that another three UK soldiers had been killed when a homemade bomb exploded and ripped through their patrol in the Gereshk district of southern Afghanistan.