Iain Duncan Smith has defended plans to get the sick and disabled back to work amid allegations that he is "punishing" society's most vulnerable.

The Work and Pensions Secretary insisted being employed was like a "health treatment" and could help make people better.

He also denied that he had a target of taking a million disabled people off benefits, arguing it was sensible to ask whether individuals could do some work rather than writing them off altogether.

In an interview with the Press Association after delivering a speech in central London, Mr Duncan Smith said: "The present system of sickness benefit has a problem. It has at its heart a test that asks a simple question - are you too sick to work or can you work full time?

"My answer is, that is the wrong question. Labour started this process and it has never quite worked in that regard.

"Quite often people want to work if they are on the sickness benefit, but they cannot work because they are not allowed to otherwise they lose their benefits.

"So we want to look at a process that allows us to be able to assess them properly, ask what can you do not just what can't you do.

"And actually then be able to say, 'look we want to get the right support for you and enable you to stay in touch with the world of work'.

"Work is actually a health treatment in a sense. Those who are in work tend to be better and those who are out of work, on sickness benefit, tend to get their conditions worse.

Asked how he would reasssure Employment Support Allowance (ESA) claimants that they would be fairly treated following heavy criticism of work capability tests, Mr Duncan Smith said: "The system will be about what can you do rather than what can't you do. That's the big change I want to make.

"I want to feel that we are saying to people, so you have a particular problem, what does that mean for you in terms of work?

"Can you do certain hours, can you do certain types of work? We will figure out what you can do and then help you get jobs that are able to do that."

Mr Duncan Smith denied that he was committed halving the number of disabled on ESA, insisting it had been Labour ministers who suggested a million claimants could be taken off the roster in 2008.

"I'm not in that prediction game. I just think the whole objective here is to save lives, to help them get better lives," he said.

In the speech - which media were not allowed to attend - Mr Duncan Smith said: "We know there remains a gap between the employment rate of disabled and non-disabled people.

"We want to ensure everyone has the opportunity to transform their lives for the better by getting into work.

"That's why, as part of our one nation approach, we have committed to halving this gap."

The work capability assessment system that determines if someone will receive the employment and support allowance (ESA) has been dogged by controversy since its introduction under Labour.

Mr Duncan Smith said ESA was supposed to reduce the number of people on incapacity benefits by one million, but since 2010 there has been a fall of just 90,000 - and plans to reform the process would now be drawn up.

Nearly one in four jobseeker's allowance claimants have a common mental health condition and need more support to find, and stay, in employment, he added.

Mr Duncan Smith said: "We need to look at the system and in particular the assessment we use for ESA - and I want to look at changing it so that it comes into line with the positive functioning of Universal Credit, and as such is better geared towards helping to get people prepared for and into what work they may be capable of, rather than parking them beyond work.

"We need a system focused on what a claimant can do and the support they'll need - and not just on what they can't do."

He added: "Nearly 11 million adults in the UK have a common mental health condition and people are much more likely to fall out of work if they do.

"We also know that being out of work for four weeks or more can actually affect people's mental health, even if the original reason for ill health was a physical one."

At a campaign rally in London later, shadow health secretary Mr Burnham will say: "It's clear that Iain Duncan Smith is now preparing a new attack on disabled people to cover for his own failures on social security.

"Talk of cutting support for people who are simply not able to work will strike fear into the hearts of many vulnerable people.

"The cruel and crude approach of the Tories has already driven many people to despair and this new drive will cause even more anxiety."

Mark Serwotka, general secretary of the PCS union, said: "The Government wants to cut the numbers of people on ESA by at least a million and appears intent on punishing those in need of support in order to meet that target.

"Iain Duncan Smith suggests that work can help people with mental health problems, and of course we agree that supporting someone into work can be a good thing. But forcing them to work through fear of losing benefit risks increasing the distress and ultimately making worse the condition of the individual."

MS Society chief executive Michelle Mitchell said: "Today's reports of further potential changes to disability benefits will be alarming for those who rely on them to get by each day. Far from writing people off, the current ESA rules mean that many people with MS are being denied the support they need.

"Any move to further tighten ESA eligibility or increase sanctioning for people with disabilities would be unacceptable.

"It's time for a common sense approach which reflects the reality of people's conditions and what stops them getting into work. The Government needs to stop talking and start listening."

Fran Woodard, director of policy at Macmillan Cancer Support, said: "Any changes to the benefits system must properly recognise the support that people need when they are ill.

"A cancer diagnosis can often be financially crippling and hit someone when they are at their most vulnerable, many will face additional costs or have to give up work to undergo gruelling cancer treatment.

"It is imperative that people with cancer only return to work when they are able to do so and have the correct support in place.

"The Government must now clarify how it will ensure any proposed changes to the benefits system will not negatively impact people with cancer."

Labour leadership frontrunner Jeremy Corbyn called for the Work Capability Assessment (WCA) to be abolished.

"Two years ago I backed calls in Parliament led by John McDonnell MP for a full cumulative impact assessment of the impact of cuts on disabled people, calls wilfully ignored by this Government," he said.

"The work capability assessment has been an unmitigated disaster for disabled people, as well as for the reputation of the DWP, its Ministers, and the contractors involved.

"I share the view of the huge coalition of DWP workers, medical professionals, disabled people's organisations, the TUC, and dozens of MPs who are calling for it to be scrapped.

"Disabled people have been targeted and are being hit up to 19 times harder by the cuts than others. The WCA has caused immense distress and even suicide, and this will only increase now that the Summer Budget announced a £30 cut per week for those disabled people deemed unable to work, but able to do some work-related activity."

Mark Atkinson, chief executive at disability charity Scope, said: "We strongly welcome the Secretary of State's commitment to supporting a million more disabled people into work.

"Disabled people are pushing hard to get jobs.

"But the gap between the employment rate for disabled people and the rest of the population has remained static for over a decade.

"The system that supports disabled people into work needs to improve, and should focus on what disabled people can do.

"We agree that the WCA should not simply be an exercise in determining what benefits someone gets.

"It should be the start of a process that gives disabled people the specialist, tailored support they need to find work. Right now, this support is just not effective enough.

"But lowering ESA isn't the answer. Reducing disabled people's incomes won't incentivise them to find a job. It will just make life harder.

"If disabled people are to find, stay and progress in work, the Government needs to provide expert, tailored support and employers need to create flexible, modern workplaces."