The proportion of UK civil servants with a declared disability has risen to 8.9%, according to new figures released by the Office of National Statistics. The figure has been climbing steadily in recent years, rising from 7.7% in 2011 – a 16% increase.

Data published on 8 October reveals that 26,498 civil servants currently have a declared disability, out of a total workforce of 439,323. Whilst total civil service numbers have fallen by more than 9,500 since 2013, the number of civil servants with a declared disability has actually risen by 58.

The detailed figures show a fall in the proportion of senior civil servants with a disability – down by 0.5 percentage points since last year to 4.5%– but increases in more junior roles led the overall proportion to rise by 0.1 percentage points.

However, the figures may not tell the whole story: many civil servants refuse or fail to declare their disability status. At HMRC, for example, some 16% of the 64,310-strong workforce have a disability – but the agency has no information on 40% of its staff. Similarly, 17.3% of the DVLA’s workforce have declared themselves disabled, but a further 28% have not told their employer whether they are or not.

Other civil service organisations with particularly high rates of disability include ACAS (19.2%), the Insolvency Service (12.7%), the Department for Education (12%) and the Department for Environment, Food and Rural Affairs (11.8%) – though high figures may be a positive sign that disabled people are confident enough to tell their employer of their status.

In some parts of the civil service, there is clear evidence of discrimination against disabled people: a June 2015 National Audit Office report, for example, found that 26% of civil servants with a long-term health condition felt discriminated against. Speaking last month at the Institute for Government, cabinet secretary Sir Jeremy Heywood raised concerns about “the harassment and bullying data we get from our people survey from people with disabilities. We really need to understand why we are getting such bad figures on that.”

Indeed, in the 2014 Civil Service People Survey, 10% of staff said they’d experienced discrimination over the previous year; of these, 9% said this was on the basis of their disability. This figure is close to the 13% who said it had been on the basis of their age and the 12% regarding their gender – yet the civil service employs much larger numbers of older people and women than disabled people, suggesting that disabled people are more likely to experience discrimination than these other groups.

Nasser Siabi, managing director of Microlink, pointed out that the numbers of disabled civil servants are not falling as the workforce contracts. Disabled officials may be staying out of voluntary redundancy programmes, he said, because they know that “the civil service is a secure place to stay, a good employer” compared to many private businesses – yet budget cuts mean that officials are struggling to access the help or equipment they need to do their jobs properly. The risk is that the civil service is left with a large cohort of disabled people who are performing weakly: “They’re reluctant to leave, but if they have problems they’re not going to give 100%,” he said. The answer, he argued, is to provide the right support to increase the motivation, morale and performance of disabled staff: “You can unlock that hidden talent.”