Not enough is being done to find almost 56,000 absconded foreign nationals due to be deported, a report has said.
The Chief Inspector of Borders and Immigration said efforts to monitor a further 80,000 foreign offenders and illegal immigrants was being "seriously compromised" by Home Office failings.
David Bolt said the number of people being monitored had created "practical difficulties" for officials involved.
The Home Office said work was already under way to address issues.
The 80,000 foreign nationals being monitored are required to report to the immigration authorities as they await possible deportation.
They include foreign offenders, people who entered Britain illegally and those who have overstayed their visas.
The 56,000 missing people includes cases dating back to 2009.
Mr Bolt's report said there were problems of "poor communication and co-ordination" between different units inside the Home Office.
This "seriously compromised" the process of monitoring them, he added.
In one instance, the inspection found that action to locate a foreign offender was only taken after 19 appointments with immigration staff had been missed.
It noted that in September 2016, about half of the "reporting population" - about 40,000 individuals - had been told to report at one of three London centres.
High numbers meant staff at the busiest centre, Becket House, had an average of two to three minutes per interview. It resulted in limited opportunities for "meaningful interactions", the report said.
The inspector has published to two reports - one on the Home Office's reporting and offender management processes and the other on its management of non-detained foreign national offenders (FNOs).
'People under strain'
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."
He said the removal of FNOs is "regularly frustrated" often by last minute legal challenges.
"Monitoring non-detained FNOs effectively is a challenge and one that raises obvious public protection concerns," he added.
The report made 14 recommendations in total, with the Home Office accepting all but two.
In response, the Home Office said it recognised the inspectors' "observations of inconsistencies in our operation" adding that it was "taking action to address this issue".
There is an ongoing review of "how we establish and maintain contact with those who have not complied with restrictions placed on their stay in the UK" the Home Office added.
The review is due to be completed by April 2018.
Immigration minister Brandon Lewis said 38,600 foreign offenders had been removed from the UK since 2010, but admitted the reports made for "difficult reading".
"People who have no right to live in this country should be in no doubt of our determination to remove them," he said.
"Our priority is to progress cases towards voluntary and, if necessary, enforced departures."
Labour MP Yvette Cooper, chairwoman of the Commons Home Affairs Committee, said the findings were "extremely concerning" and undermined the credibility of "the whole immigration system".
She said: "For the Home Office to have simply lost track of thousands of foreign national offenders and people here illegally is just not good enough."