The number of suicide attempts in immigration removal centres is at an all-time high, averaging more than one a day, according to official figures.

The new data from the Home Office comes as the government faces growing pressure to reduce the use of immigration detention, especially for vulnerable individuals; people who have committed no crime; and those with no identified risk of absconding. Concerns have also been expressed about the cost to taxpayers of detaining more than 30,000 people a year.

The figures, which were released following a freedom of information request by the NGO No Deportations, showed there were 393 recorded suicide attempts in 2015, up 11% on the previous year.

The number of people entering detention last year (32,466) also increased – up 7% on 2014. But 3,515 more people left detention in 2015 compared with the previous year.

The figures showed that Harmondsworth immigration removal centre near Heathrow had the highest number of suicide attempts, 105, followed by Yarl’s Wood with 64. Yarl’s Wood in Bedfordshire mainly houses female detainees.

A total of 2,957 detainees were on suicide watch during 2015 including 11 children.

An independent review into the welfare of immigration detainees has called for a significant reduction in the number of people held in immigration detention. The review, led by the former prisons ombudsman Stephen Shaw and published in January, made particular reference to pregnant women who it said should not be detained at all. In an interview with the Guardian last week one woman, who was detained in Yarl’s Wood while pregnant, said she was in a state of acute distress and felt very unwell.

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Commenting on the figures released on Monday, Shaw said: “Levels of self-harm are critical indicators of the health of any institution and the welfare of those in detention. Many detainees are extremely vulnerable and experience high levels of anxiety and depression. If implemented, the proposals I have made to the government would improve welfare, reduce vulnerability, and ensure a more effective use of the detention estate.”

A spokeswoman for the charity Medical Justice, which works to improve the health of immigration detainees, said: “We find that many detainees have serious mental health issues, having suffered torture in the home countries and then faced with the trauma of indeterminate detention. The increasing numbers of detainees who self-harm or attempt suicide is very concerning and the healthcare provided in immigration removal centres is not adequate to deal with the levels of mental distress.”

Shaw said there should be a presumption against detention of victims of rape and sexual violence, people with learning difficulties and those suffering from post-traumatic stress disorder. However, detainees with one or more of those issues continue to be detained. The Home Office has confirmed that in response to Shaw’s report an adults at risk policy will be published next month.

On Monday, a Home Office spokesperson said: “We take the welfare of our detainees very seriously. That is why the home secretary commissioned Stephen Shaw CBE to carry out an independent review into the policies and operating procedures that impact on detainee welfare. As a result of his findings, we are adopting a policy whereby all decisions on immigration detention will consider whether an adult is at risk.”

The spokesperson added that a mental health action plan would be published and that a new approach to detainee case management would be implemented. “We expect these reforms – and broader changes in legislation, policy and operational approaches – to lead to a reduction in the number of detainees and the length of time they spend in detention before removal.”