Whitehall was accused of “institutional discrimination” yesterday after it emerged black civil servants are far more likely to be categorised as underperformers.

Data supplied by government departments also shows that disabled and older mandarins are valued less than their colleagues.

Black and ethnic minority workers make up 27 per cent of the lowest performance category — and only 10 per cent of the workforce.

In Vince Cable’s business department, 30 per cent of disabled workers received the lowest performance category, compared to just 17 per cent of able-bodied staff and those who did not declare their status.

And 12 per cent of over-50s at the Department for Communities and Local Government, headed by the 62-year-old Eric Pickles, were deemed underperformers compared to just 7 per cent of younger colleagues.

Civil Service union Prospect deputy general secretary Leslie Manasseh said: “Either our BME and disabled colleagues or those over 50, tend to be low performers, or the system is working against them. We’re clear that the system is at fault.

“Too many performance markings are driven by the need to match pre-determined quotas rather than being a genuine reflection of performance. 

“Prospect has sympathy with line managers who are under pressure to deliver these quotas — particularly when it’s on the basis of a nod and a wink from above.

“The Civil Service must ditch forced rankings and build in checks and safeguards to ensure that staff with protected characteristics are treated equally.”

Ms Manasseh called for departments to “equality-proof” their performance assessments and provide equality training for managers.