Iain Duncan Smith says that disability benefits reform are needed.

Planned changes to disability benefits will go ahead, the Work and Pensions Secretary Iain Duncan Smith has said, insisting that reform is needed. It has emerged half a million people are to lose the Disability Living Allowance over the next four years. But Mr Duncan Smith told the Daily Telegraph the number of people claiming it had risen by 30% in recent years, with many people "allowed to fester". Labour said he was approaching reform with "contempt and carelessness".

Disability Living Allowance can be claimed by people with a physical or mental disability. This has to be severe enough that they need help caring for themselves or have difficulty walking. It can be claimed irrespective of employment status, and is not usually affected by savings or income. The government reportedly wants to cut at least a fifth off the £13bn annual bill. Under the changes, two million claimants would be reassessed in the next four years, with only those considered to be in need of support qualifying.

Mr Duncan Smith said the cost had "been rising well ahead" of illness, sickness, disability and other "general trends in society", adding: "A lot of that is down to the way the benefit was structured so that it was very loosely defined." He added: "Something like 70% had lifetime awards, [which] meant that once they got it you never looked at them again. They were just allowed to fester."

The allowance is claimed by more than three million people - but it is due to be replaced with a Personal Independence Payment, which ministers say will focus on those most in need. The government is consulting on new eligibility criteria, to be announced in the autumn. Campaigners have warned that assessments must be fair, otherwise people with serious disabilities could lose the money to cover costs such as maintaining wheelchairs and using specially adapted cars.

Deputy Prime Minister Nick Clegg said Disability Living Allowance costs had "just ballooned out of all proportion". He added that, without change some people "might receive the support they no longer need, and it might also mean that some people receive less than they deserve... [The change] is also about people getting more support than they do at present."

For Labour, shadow work and pensions secretary Liam Byrne said: "This government has approached the vital reform of disability benefits with a mix of contempt and carelessness. It is totally unacceptable that Iain Duncan Smith is refusing to apologise for accusing disabled workers of 'not doing any work' and 'just making cups of coffee'. Meanwhile, his department is getting wrong nearly half of disabled people's requests for help and his Work Programme is totally failing to offer disabled people a thing. If Iain Duncan Smith dropped the bluster of yet another relaunch and actually tried running his department properly, we might make some progress."