More than 400 migrants were illegally detained at removal centres, internal government emails have revealed, according to a local media report.

Home Office emails obtained by the BBC said their detention was “no longer legal,” and authorities were required to move them “to hotels ASAP.”

The detention centres were used to keep hundreds of migrants – described in one email as “overflow” – from the migrant processing facility at Manston, a former military base in the city of Kent, which was closed after human rights groups brought a legal challenge about the conditions there.

“The email was sent on 4 November, the day after Home Secretary Suella Braverman visited Manston amid pressure to get a grip on overcrowding at the site,” the BBC reported.

The Home Office said a massive inflow of small boat arrivals had put “huge pressure” on the asylum system, the report said.

READ MORE:UK to relocate asylum seekers to Rwanda from mid-June

'Chaotic failure'

Asylum seekers who arrive in the UK are kept in processing centres for 24 hours, after which they are usually moved to hotels.

The department said it had a legal duty to house asylum seekers who would otherwise be destitute, and officials had “worked tirelessly to move people into hotels or other accommodation as quickly as possible.”

Shadow Home Secretary Yvette Cooper said the emails “demonstrate the Home Office knew that they were breaking the law and lay bare how their chaotic failure to plan led to this situation.”

“Under [Braverman’s] direction, the Home Office has lost control of the system and has created potential substantial legal costs for the taxpayer,” she said.

The latest reports come amid the UK’s push to implement its controversial policy of sending asylum seekers to Rwanda, which overcame a legal challenge this week as London’s High Court ruled that the plan to deport people to the African country is lawful. 

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