Labour’s shadow chancellor has called for Iain Duncan Smith to face criminal charges over allegations that his failure to address a coroner’s concerns about the “fitness for work” test led to the deaths of disabled benefit claimants.

John McDonnell (pictured) made the call during a speech to the TUC Disabled Workers Conference in London on Friday (19 May), and his backing was reportedly greeted with enthusiasm by the audience of disabled trade unionists.

The Scottish-based grassroots group Black Triangle, backed by many other disabled activists, has led calls for the former work and pensions secretary to face a criminal investigation for misconduct in public office following his apparent refusal in 2010 to address a coroner’s concerns about the safety of the discredited work capability assessment (WCA).

They want to hold Duncan Smith and his former employment minister Chris Grayling to account in court for their failure to improve the safety of the WCA, even though they were warned that it risked causing further deaths.

Scottish police are currently assessing whether to launch a criminal investigation, after an approach from Black Triangle’s co-founder John McArdle.

McDonnell backed the efforts of Black Triangle on Friday, and later confirmed his position to Disability News Service through a spokeswoman, who said: “Black Triangle are taking action against Iain Duncan Smith and what he said [at the conference] was that he would support that action.”

Just hours after McDonnell made his speech, the conference voted to send a motion it had unanimously approved – calling for the WCA to be scrapped and replaced “by a fair and humane process” – to the TUC’s annual congress in September, selecting it out of 22 motions discussed by delegates during the two-day conference.

The WCA motion, proposed by the Communication Workers Union (CWU), refers to government-funded research which concluded last November that the programme to reassess people claiming incapacity benefit through the WCA could have caused 590 suicides in just three years.

It also highlights the letter written to the Department for Work and Pensions in 2014 by coroner Mary Hassell, following an inquest into the death of a north London man, which found that his decision to take his own life had been triggered by being found fit for work.

The motion says: “Conference believes that enough is enough and that the time has come for action before more lives are lost.

“The DWP and government have been brought into disrepute and must be held to account.”

Jonathan Bellshaw, the CWU delegate who proposed the motion, told the conference that there was “damning evidence” of the links between suicides and the WCA.

He said: “One death linked to the WCA is one too much. 590 deaths… shows a government not fit for purpose.

“In some countries, that would be cause for a government to be removed by a vote of no confidence.

“Here in the UK, the government continues to ignore all this and ignores clear evidence that their policies are killing people. That’s the truth.”

Another delegate, Hilary Mellor, from Unison, added: “The fact that several hundred suicides have been linked to the WCA is totally unacceptable, unjustifiable and immoral. It is imperative that we stand together to fight this injustice.”

McArdle said after the conference that securing McDonnell’s support for a prosecution was “absolutely amazing”.

He said: “We warmly welcome the support of the shadow chancellor John McDonnell.

“John has been an outstanding supporter of the disabled people’s movement, especially since the foundation of the Black Triangle campaign and Disabled People Against Cuts (DPAC) in 2010.

“We believe the reason he is able to stand by us is because our complaint to Police Scotland is soundly founded in law.”

Linda Burnip, co-founder of DPAC, said: “DPAC are delighted that John McDonnell fully supports Black Triangle’s attempts to bring Iain Duncan Smith and Chris Grayling to account for their failure to act when the deaths of thousands of disabled claimants could have been avoided.”

Meanwhile, SNP MP Deidre Brock has told ministers during a House of Commons debate that they should listen to Black Triangle, and that its attempt to bring Duncan Smith and Grayling to court could “commend the ingredients of the government’s poisoned chalice to their own lips”, which she said would be “even-handed justice”.

Disabled activists have also welcomed the decision of the Cannes film festival jury to award its Palme d’Or award for best film to Ken Loach’s I, Daniel Blake, which follows a man who is found fit for work through a WCA after a heart attack, but faces destitution because he is too ill to work.

Black Triangle – including its medical adviser Dr Stephen Carty – advised scriptwriter Paul Laverty on the WCA and how DWP’s sanctions system works.

McArdle said: “I can only say we feel absolutely elated that it has won to such enormous fanfare.

“This is going to break hearts of stone and will – without any doubt – change minds and direct hearts towards our cause with empathy and compassion.

“It is a magnificent win for all of us in our collective movement for justice and respect for our fundamental human rights.”