An asylum centre that became dangerously overcrowded last year could again become overwhelmed within weeks, civil servants and union representatives have told the Guardian. Staff working at the Manston processing centre have said there could be a return to conditions that led to disease and violence because of the lack of “upstream accommodation” for asylum seekers once they have been processed. Staff are also concerned about the safety of working on the site amid the removal of asbestos that is yet to be completed, sources said. In a further development, hundreds of asylum seekers who were illegally held at the centre in November are bringing claims against the Home Office for unlawful detention and inhumane treatment. The cases could result in payouts of millions of pounds. Manston was supposed to hold up to 1,600 people seeking asylum for short periods while they underwent security checks. But conditions quickly deteriorated in the autumn after the numbers rose to 4,000. There were cases of infectious diseases including diphtheria and outbursts of violence. Hopes were raised that potential overcrowding this year would be avoided by government moves to increase accommodation for asylum seekers. The immigration minister, Robert Jenrick, announced in March that long-trailed plans for camps on former military sites would go ahead, as well as using barges and cruise ships. However, many of the sites may not be ready to accept asylum seekers for weeks amid legal challenges and local protests. A Whitehall source said concerns had been raised with the department about a possible sharp rise in the numbers of people coming to the UK by small boats in June and July. “The numbers coming could jump and the Home Office won’t have the capacity at Manston and won’t have anywhere else to send the overflow,” they said. Lucy Moreton, a professional officer at the ISU union, which represents border force staff, said: “We are concerned about the possibility of a return of problems at Manston if the government struggles to find accommodation upstream. It will take time to address the various blockages to bring floating accommodation and military bases into use, and the numbers of people coming by small boats are increasing.” Staff working at Manston have confirmed that there is ongoing work to remove asbestos at the site. Sites identified by the Home Office as asylum accommodation are former RAF bases at Scampton in Lincolnshire, Wethersfield in Essex, and Catterick in North Yorkshire, as well as the site of a former prison in Bexhill, East Sussex. The government has also outlined plans to use a barge in Portland off the Dorset coast. The government is facing judicial reviews challenging the use of Scampton and Wethersfield to house asylum seekers from local authorities, and neither site is expected to be ready before July, sources said. The first 400 asylum seekers due at Bexhill are not expected before September. More than 200 asylum seekers – single adults, families and lone children – are bringing legal cases because many were detained at Manston for more than the 24-hour limit that was in place at the time. Recently released Home Office data shows that for 20 days between 11 October and 6 November 2022 the numbers on the site exceeded 1,600 – peaking at 3,965 on 30 October 2022. Between 15 August and 23 November last year, more than 18,000 new arrivals were processed through Manston, according to the Home Office. Concerns about conditions between September and November last year include: Safeguarding issues around people who did not know one another sleeping close together. Blankets being used to block wind and rain from coming into tents. Contents of portable toilets overflowing during a rainy period and seeping into tents. Children wearing inadequate clothing. Asked about the legal cases, Home Office sources said: “It would be inappropriate to comment or speculate on the potential outcome of ongoing legal proceedings.” The Home Office hopes to avoid being sued in future after introducing a new category of detention centre, which the government has labelled a “residential holding room”. A regulation was introduced in December that says people can be detained for 96 hours, or longer if authorised by the home secretary in “exceptional circumstances”. A Home Office spokesperson said: “We continue to work across government to deliver cheaper and more suitable accommodation for those arriving on small boats, including through surplus military sites and vessel accommodation. In addition, we are committed to processing individuals through Manston as quickly as possible and have introduced residential holding rooms onsite to ensure we have sufficient capacity when there are large numbers of arrivals. “But the best way to relieve these pressures is to stop the boats in the first place. That is why we have introduced legislation which will ensure people arriving illegally in the UK are detained and swiftly removed to another safe country.”