Opponents have accused Iain Duncan Smith of "striking fear into the hearts" of the sick after he unveiled plans to get one million more claimants off benefits.

The Conservative Work and Pensions Secretary said that too many people with common mental health problems were out of work.

Getting back into employment would bring health and other benefits, he said, as he signalled plans to reform sickness assessments.

The SNP accused Mr Duncan Smith of targeting the most vulnerable in society with an “ideologically-driven attack” on the social security system while Labour accused him of terrorising the genuinely ill in an effort to "cover" his party's own failures on welfare reform.

But Mr Duncan Smith said that his policy would help those affected.

"There is one area on which I believe we haven't focused on enough - how work is good for your health," he said.

"Work can help keep people healthy as well as help promote recovery if someone falls ill.

"So, it is right that we look at how the system supports people who are sick and helps them into work.

"Let me be clear: a decent society should always recognise that some people are unable to work as a result of physical or mental ill health - or both.”

Under the current system a capability assessment determines if claimants receive employment and support allowance (ESA).

Mr Duncan Smith said that ESA had been designed to reduce the number of people on incapacity benefits by one million but that since 2010 the numbers had fallen by just 90,000.

Plans to reform the system will now be drawn up, he added.

Richard Kramer, from the charity Sense, which supports deafblind people, said: “We welcome the idea that disabled people should be better supported to find work.

"However, we are looking at this from the wrong end of the telescope and risk making problems worse for disabled people.

"The fault line is often not the benefits system but ensuring that we dismantle the many barriers that prevent people getting opportunities to work in the first place."

The SNP's social justice and welfare spokesman at Westminster Eilidh Whiteford said: "Ian Duncan Smith’s speech is his way of trying to hide further benefit cuts and is yet another example of the Tories ideologically-driven attack on our social security system.

"Once again, it is society’s most vulnerable who will suffer most as a result."

Labour's shadow health secretary Andy Burnham said: "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 party's shadow Scottish secretary Ian Murray said:“The Tories’ attacks on sick and disabled people are ideological and morally reprehensible".