Thousands of asylum seekers have dropped off the Government's radar, an official report has revealed.
Caseworkers told inspectors there were around 10,000 cases where the main applicant and any dependants, such as children, were not in contact with the Home Office or had absconded.
The number will include those who were awaiting a decision, as well as those who are still in the country after their application has been refused.
While enforcement teams could conduct residential visits to attempt to trace missing claimants they were "reluctant to do so as this work was not a priority and was considered a drain on resources", a report by the borders watchdog said.
It was also disclosed that, as of September last year, there were more than 30,000 failed asylum claims where individuals had not been removed from the country or given leave to stay more than two years after their right of appeal had lapsed or been exhausted.
A report by the chief inspector of borders and immigration David Bolt said: "Failure to deal with asylum cases in a timely manner was inefficient as well as ineffective.
"The more time an asylum case took to resolve, the more likely barriers to removal would arise from the formation of relationships, the birth of children and other community ties.
"It also meant individuals were left not knowing if or when the Home Office might take action to remove them."
Immigrants who are required to report to officials are recorded to have absconded when they fail to attend.
In a sample of 338 cases examined by inspectors, 48 individuals were logged as absconders.
Of these, an attempt to locate the person had been made in only nine instances, including five in which teams visited last known addresses.
On the 39 occasions where no attempt was made to trace the immigrant, six records indicated this was because of resources or because the case did not meet priorities at the time. In the other 33 cases, no explanation was recorded.
Staff at sites which some immigrants are required to attend periodically said their resources have become increasingly stretched due to the number of individuals on reporting regimes, which stood at 47,000 last December.
The report said: "Managers said that the numbers reporting placed pressure on back office administrative functions, such as taking action against individuals who had failed to report."
It added that limited bed space in detention centres meant some individuals who had repeatedly failed to abide by the rules were not pursued or detained because they did not fit "priority categories", with some of those later absconding.
The inquiry identified a "disconnect" between the work of different departments, saying poor communication was having an "adverse impact on efficiency and effectiveness".
However, the report said increases in the numbers of voluntary departures suggested the Home Office's focus on this area was having an impact.
A second report published today said intelligence about illegal working mostly consisted of "low-level allegations" by members of the public.
Inspectors found this had led immigration enforcement to focus on high street restaurants and takeaways, adding: "Other business sectors and possibly other nationalities had been neglected by comparison."
There are no reliable estimates for the numbers of migrants working illegally in the UK, the watchdog added.