The Home Office has lost touch with more than 10,000 asylum seekers in Britain and efforts to trace them are considered “a drain on resources”, the official immigration watchdog has revealed.

A report on immigration removals by David Bolt, chief inspector of borders and immigration, also discloses that the rising number of people required to report to the immigration authorities – 47,000 as of December 2014 – is increasingly stretching enforcement resources.

A second report by the chief inspector, looking at measures to tackle illegal working, cites “unfair and unreasonable treatment” of individuals during one large-scale immigration raid on a commercial laundry in north London. Some of the laundry’s workers, mainly women, were left standing for five hours while others were questioned. They had access to a toilet but were left without food. During the operation one Polish-born British woman was questioned for a total of more than two and a quarter hours before being cleared.

The chief inspector’s report also criticises the concentration of immigration raids on high-street Indian and Chinese restaurants and takeaways, saying these nationalities were the hardest to remove because of difficulties in obtaining travel documents. A 184-strong sample of immigration enforcement visits carried out in 2014 included 107 high-street restaurants or takeaways.

Bolt said that immigration enforcers would find it difficult to defend challenges maintaining they were unfairly focusing on particular businesses or sectors.

The inspection report also reveals that later in 2014 Home Office tactics switched from raiding restaurants and takeaways, seeking out illegal workers, to carrying out more “educational” visits to encourage employers to comply with the law on illegal working. Bolt said it was hard to assess the effectiveness of the new approach but the Home Office claimed it had led to more people voluntarily leaving the country.

The two reports confirm that there has been a significant increase in the number of people voluntarily leaving the UK with the total of “notified voluntary departures” rising 26% to 12,266 so far in 2015. But they also document serious weaknesses in the Home Office systems for keeping track of certain asylum seekers and those staying illegally in Britain, with the numbers of enforced removals dipping over the past three years.

The chief inspector found that, as of September 2014, there were 30,406 asylum seekers who had neither been removed nor been given permission to stay two years after their appeal rights had been exhausted. In a sample of 67 cases the inspectors found that 22 had been recorded as absconders.

“Asylum casework managers told us that there were approximately 10,000 asylum claims where the claimant and dependants were not in contact with the Home Office or had absconded. While enforcement teams could conduct residential visits to attempt to trace absconders they were reluctant to do so as this work was not a priority and was considered a drain on resources,” reported the chief inspector.

He added: “Reporting centre staff told us that their resources were becoming increasingly stretched due to the number of individuals on reporting restrictions, which stood at around 47,000 in December 2014. Managers said that the numbers reporting placed pressure on back-office administrative functions, such as taking action against individuals who had failed to report.”

The chief inspector also reveals that the special family accommodation near Gatwick, named Cedars, which was set up for those facing deportation, has only been used for 14 families in the past year at a cost of more than £6m. The Home Office is now reviewing this immigration enforcement unit.

An official Home Office response to the removals report said the department had continued to focus on arresting and removing those not given permission to stay on in the UK but had also transformed the approach to cover a much wider range of cases. “This means there are now real consequences for those entering the country unlawfully or failing to comply with the permission given to stay.”