The fight against terrorism is being undermined by the failure to recruit more ethnic minority staff at GCHQ and allegations of racism at the security services' listening centre, a leaked Whitehall report has warned.
The shortage of officers fluent in such languages as Urdu and Arabic is hampering its ability to eavesdrop on telephone calls between suspected terrorists, to tap into their emails and monitor website visits.
Conversations between extremists – often thousands of miles apart – tend to contain coded references and "street" slang whose significance needs to be recognised by a skilled speaker.
A spokesman for GCHQ – where just 2.5 per cent of staff are from an ethnic minority background – said urgent efforts were under way to boost numbers.
The private internal report said it was "critical to have a diverse staff group who are able to profile and recognise certain behaviour patterns and communications."
It warned: "There is a very small pool of black and minority ethnic employees within the total workforce.... Specific concerns have been raised by both management and staff around the language team."
Allegations of discrimination emerged during an inquiry which was authorised by Sir Gus O'Donnell, the head of the Civil Service.
It recorded GCHQ officers from ethnic minorities complaining that their loyalty to Britain was constantly questioned and that they were expected to work harder, and for less reward, than their white colleagues.
Upon applying for jobs, they said they faced "inappropriate, insensitive and offensive" questions during vetting about their backgrounds.
Most of the estimated 125 ethnic minority staff at GCHQ, which employs 5,000 people at its Cheltenham complex, work in relatively junior posts. All of its 60 senior civil servants are white and just a handful of middle managers black or Asian.
The security services have a policy of requiring recruits to hold a UK passport and have at least one British parent.
Despite the rule, MI5 has said 8 per cent of its staff are from an ethnic minority, close to the proportion in the general population, following a recruitment drive. MI6 has followed suit.