Anne Owers, outgoing chair of the police watchdog, says the relationship between ethnicity and use of force must be looked at closely

The official police watchdog says it is concerned about a rise in the proportion of ethnic minority people who have died after police use of force or restraint in the last eight months, the Guardian has learned.

Of the 11 people who have died after a clash with police in England and Wales since April 2017, six are from an ethnic minority background, says the Independent Police Complaints Commission (IPCC).

One case, involving a woman in Cheshire, involved the use of leg shackles. The full circumstances of all 11 cases are still under investigation either by the IPCC or at inquests.

Dame Anne Owers, chair of the IPCC, told the Guardian that the watchdog had asked for the data on the cases. She said: “Of those, five men were black or mixed race, one was a black woman and the remainder were white.”

In 2015-16, of 11 deaths after police restraint or use of force, three were from ethnic minorities, while in 2016-17, five ethnic minority people died out of 15 deaths after the use of force or restraint, the IPCC said.

Owers said: “We need to look closely between the relationship between ethnicity and the use of force.”

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The IPCC says more work is needed to understand why the deaths have taken place, and that reasons could include use of alcohol, drugs or the increasing number of people with mental health problems whom police have to deal with.

Campaigners claim prejudice may play a role, and discussion around the issue in the UK is influenced by the debate in the US, which has been driven by the Black Lives Matter movement.

The latest figures, covering the last eight months of 2017, suggest a growth in racial disproportionality, so that the proportion of ethnic minority people who have died exceeds their proportion in the overall population.

The incidents include high-profile cases such as Rashan Charles and Edson Da Costa in London, whose deaths led to tension in the streets between police and communities. In these two cases, the IPCC has decided officers should be issued with notices warning that their conduct is under investigation.

The IPCC has been much criticised and its release of the emerging data may prove controversial at a time when some police officers feel that their ideals of public service are under unwarranted attack.

Concerns about police and race are longstanding in some communities, and have been a constant theme of Theresa May’s time as home secretary and prime minister.

In October, a report into deaths in custody ordered by May showed a possible racial factor. The report, by Dame Elish Angiolini, said: “Deaths of people from BAME communities, in particular young black men, resonate with the black community’s experience of systemic racism, and reflect wider concerns about discriminatory over-policing, stop and search, and criminalisation.”

A report by David Lammy for the government in September found racial disparities across the criminal justice system.

Owers said: “We will need to see if there are the same patterns that Lammy found, whether they apply here.”

Owers also revealed that the government had promised to introduce a requirement that police officers cooperate with inquiries into their conduct. In the past, some have refused to answer oral questions in interview and instead provided written answers at a later date.

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Owers said: “There should be a duty to cooperate with an independent investigation.”

She said the duty of cooperation, similar to the duty of candour introduced in the health service, had been promised in forthcoming legislation and was needed because “sometimes we have found a defensive approach”.

“We are assured that is going to happen, in the new regulations in the new Policing and Crime Act.”

As an example, Owers said that when the IPCC got the power to require officers to attend an interview, the Police Federation advised its members, who are rank and file officers, to answer questions in writing. That slowed down investigations by the IPCC, which police then complained about, Owers said.

The IPCC ceases to exist this weekend, and from Monday will become the Independent Office for Police Conduct, with a new leader and stronger powers. Owers is standing down after five years in charge. In a number of cases, both the police officers under investigation and the families who had lost loved ones were united only in criticising the IPCC.

The IPCC was set up as part of the reforms following the public inquiry into the Stephen Lawrence case. It was supposed to be independent and gain public confidence.

Owers accepted it had faced criticism from all sides.

Deborah Coles, the executive director of Inquest, a charity that gives specialist advice in cases of deaths in state custody, said: “These latest shocking figures show this is a systemic problem, exposing the structural racism embedded within policing and criminal justice. Deaths continue because of failing systems of investigation, oversight and accountability.

“No longer can the government ignore the serious human rights issues raised by this deadly, disproportionate policing.”

The National Police Chiefs’ Council lead for restraint and self-defence, Commander Matt Twist, said: “The police service has made great progress in making use-of-force recording more accessible and transparent, with police forces now regularly publishing data on their officers’ use of force. This data will help us to ensure that our training, tactics and equipment are fit for purpose and that when force is used, it is used legitimately.”

Those people of an ethnic minority background who died after police use of force or restraint in the last eight months were Rashan Charles, Edson da Costa, Darren Cumberbatch, Shane Bryant and Nuno Cardoso. A black woman in her 50s, who has not been named and on whom leg restraints had been used, died several days after contact with Cheshire police.

The other people who died were Gary Williams in Merseyside, Douglas Oak in Dorset, Mark Cole in Devon and Cornwall, David Molloy in Greater Manchester and a man, yet to be named, who died after an incident involving the Met.