Home Office officials have admitted that “we completely lost our grip” on the situation at a notorious asylum processing centre that led to 18,000 people being unlawfully detained in horrific conditions.

Overcrowding at Manston, a former RAF base in Kent, in autumn 2022 led to an outbreak of diphtheria and scabies. Asylum seekers who had crossed the Channel in small boats were forced to sleep on filthy floors or on flattened cardboard boxes, while toilets were overflowing with faeces. Women and children were forced to sleep close to unrelated men and there were claims of assaults by guards.

Government documents disclosed in the high court last week reveal that the situation at Manston was much worse than news reports originally suggested. But the court also heard that uncovering the truth may prove difficult because “it appears no steps have been taken to preserve and obtain relevant WhatsApp messages”.

Three former home secretaries, Priti Patel, Grant Shapps and Suella Braverman, as well as two ex-prime ministers – Boris Johnson and Rishi Sunak – may now have to appear at an official inquiry.

Asylum seekers were supposed to be held at Manston for no more than 24 hours, but the new documents show that 18,000 people – out of a total of 29,000 processed between June and November 2022 – were detained there for much longer.

The government may now be forced to pay out tens of millions of pounds in compensation.

Manston opened to process small boat arrivals in January 2022. By the summer of that year, however, conditions had deteriorated sharply as better weather for Channel crossings led to more and more arrivals. The situation became critical in the autumn of 2022, when the site, which was designed to hold a maximum of 1,600 people, was accommodating 4,000.

On 19 November, Kurdish asylum seeker Hussein Haseeb Ahmed, who was processed at Manston, died in hospital after contracting diphtheria.

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The previous government agreed to hold a statutory inquiry into what went wrong at Manston but in September the new home secretary, Yvette Cooper, decided to downgrade it to an independent one that has fewer powers to compel witnesses such as former home secretaries to attend. She cited the projected costs of the planned inquiry – about £26m – as a reason for downgrading it to a much cheaper inquiry focusing on a review of documents, with an estimated cost of £2.6m.

In a legal challenge against this decision that opened in the high court last week, new information was disclosed for the first time by the government, revealing the scale of what went wrong at Manston in the period from 1 June to 22 November 2022.

The documents also show that Home Office staff had no reliable data about Manston between September and November 2022 “as we completely lost our grip on it”.

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Lawyers for the asylum seekers bringing the high court challenge said in their written submissions: “It appears to be uncontroversial that there was large-scale law-breaking by the defendants’ department [the Home Office]”, adding that multiple members of staff are involved in “allegations of systemic failure” and that there is a need to examine “questions of management and institutional culture”.

One of the main questions the lawyers hope the inquiry will try to answer is whether either Patel or Braverman gave an order to stop transferring newly arrived asylum seekers from Manston to hotels because of criticism over the use of expensive accommodation to house them – a decision that led to the overcrowding crisis at the processing centre.

According to documents submitted to the court: “In June 2022 operational commanders at Manston were told that ‘the hotel pipeline has been switched off’.”

Lewis Kett, a solicitor at Duncan Lewis, who is representing some of the asylum seekers in the case, said: “The numbers likely detained unlawfully at Manston are entirely unprecedented and the conditions they were held in were deplorable.

“An independent inquiry will need the powers and resources to properly understand how this happened to learn lessons and prevent an incident like this occurring again.”

Home Office sources said they do not comment on continuing legal proceedings. Patel and Braverman were approached for comment.