Thousands of asylum seekers have dropped off the immigration authorities' radar and efforts to trace them are seen as a "drain on resources", a watchdog has revealed.
Caseworkers told inspectors there were approximately 10,000 cases where the claimant and dependants including 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 teams could conduct residential visits to attempt to trace missing individuals they were "reluctant to do so as this work was not a priority and was considered a drain on resources", a report 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.
The 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."
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.
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.
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.
However, the report concluded that increases in the numbers of voluntary departures suggested the Home Office's focus on this area was having an impact.
The department said absconders are traced and contact is re-established every day.
A Home Office spokesman said: "We expect people with no right to be here to leave the country voluntarily -- and we offer help for them to do this - but where they do not, we will enforce their departure.
"The report rightly links our efforts to make it harder to stay illegally in the UK with increased numbers of voluntary returns.
"Results show that the total number of voluntary returns was over 50,000 during the last two years, including over 27,000 notified and assisted voluntary departures.
"In the same period we enforced the removal of over 20,000, giving a total of over 79,000 removals. These numbers include some of the 25,000 foreign national offenders successfully removed since 2010."
Measures such as limiting access to bank accounts and imposing NHS charges make it harder for illegal migrants to stay, while the new Immigration Bill will strengthen powers, the spokesman said, adding: "Since the inspections we have already taken significant steps to address the Inspector's findings."
A second report said intelligence about illegal working mostly consisted of "low-level allegations" by members of the public.
Inspectors found this had resulted in a focus on high street restaurants and takeaways, adding: "Other business sectors and possibly other nationalities had been neglected by comparison."
It also disclosed that enforcement officers are not allowed to pursue people seen leaving a premises during raids unless they are aware of their immigration status.