Tens of thousands of foreign nationals including convicted criminals and illegal immigrants have disappeared off the Home Office's radar, it has emerged.
At the end of last year, there were nearly 56,000 "declared absconders" - meaning authorities did not know their whereabouts, two reports from the immigration watchdog said.
More than 700 foreign national offenders (FNOs) were recorded as having absconded after being released into the community.
The assessments detail how about 80,000 people who are liable to be removed from Britain are required to report to officials regularly rather than being held in detention while their case is determined.
The number includes those who have entered the country unlawfully, breached their original conditions of entry, face deportation as a result of committing a crime, and asylum seekers.
Ministers acknowledged that parts of the reviews published by Chief Inspector of Borders and Immigration David Bolt made for "difficult reading".
The reports warn there is "little evidence" that effective action is being taken to locate the vast bulk of absconders; reveal foreign criminals can fail to attend meetings with staff on as many as 19 occasions before the alarm is raised; and detail how nearly a third of planned removals of criminals failed, with attempts often frustrated by last-minute legal challenges
One inspection examined the effectiveness of the Home Office's management of the "reporting population" - those who are notified of their liability for detention and removal from the UK.
The second focused on the regime for non-detained foreign national offenders towards their removal from the UK.
Mr Bolt said: "In both cases, I found people and processes under strain.
"The numbers required to report routinely mean that it is extremely difficult for staff at reporting centres to ensure that reporting events are 'meaningful', in terms of encouraging voluntary departures or resolving barriers to removal.
"Meanwhile, the removal of FNOs is regularly frustrated, often by last-minute legal challenges, and monitoring non-detained FNOs effectively is a challenge and one that raises obvious public protection concerns."
If there is no immediate prospect of deportation or removal, a foreign offender who has completed their custodial sentence may be released into the community.
The most recent official figures showed there were 5,728 FNOs who were subject to deportation action living in the community.
Inspectors flagged up "major inconsistencies" in the handling of failures to report to authorities by criminals and other overseas nationals.
In two cases, FNOs had failed to show up on 13 and 19 consecutive occasions before"absconder action" was launched.
In 2016/17, of 9,288 planned removals of FNOs, 6,182 (67%) were successful, while 3,106 (33%) failed.
Caseworkers expressed frustration that factors outside their control made it difficult to remove overseas offenders.
Difficulties in obtaining travel documents can hinder removal attempts and there is "considerable evidence" of individuals making last-minute asylum claims, lodging judicial review applications and in some cases avoiding enforcement teams to frustrate planned removals, according to the report.
Immigration minister Brandon Lewis said: "People who have no right to live in this country should be in no doubt of our determination to remove them.
"Our priority is to progress cases towards voluntary and, if necessary, enforced departures and we have removed more than 38,600 foreign offenders since 2010.
"This week, like every week, more than 100 foreign criminals will be removed from the UK.
"Elements of these reports make for difficult reading, but we are committed to a programme of transformation.
"That is why we are conducting a fundamental review of how we establish and maintain contact with those who have not complied with restrictions placed on their stay in the UK.
"Our future operations will be shaped by the outcome of this review."