Border officials have lost track of a population of asylum seekers and migrants the size of Cambridge, a critical report by MPs has said.
UK Border Agency figures showed the number of "lost" cases had tripled in six months from 40,500 in March to 124,000 in September.
The so-called controlled archive had become a "dumping ground for cases where the UK Border Agency has lost track of the applicant", MPs on the Commons Home Affairs Select Committee said.
The "controlled archive", the MPs said, was "a bureaucratic term which hides the true nature of a Government department's activity and is designed to deflect attention away from it".
It would more appropriately be called "an archive of lost applicants", they said.
The archive includes cases in which around 98,000 asylum seekers cannot be found and the agency has no idea whether or not the applicant even remains in the UK, legally or otherwise.
It also includes around 26,000 migrants following a review of cases, most of which are more than eight years old, and involve those who have overstayed their visas or who have been refused an extension of leave, such as students.
The MPs said: "Whilst we appreciate the difficulties involved in tracing people with whom the agency have lost contact, usually for a period of several years, it is clear that the controlled archive has become a dumping ground for cases on which the agency has given up.
"The controlled archive has increased significantly as the deadlines for the legacy backlog and the migration case review have approached.
"From 18,000 files in November 2010, the archive now contains 124,000 files, roughly equivalent to the population of Cambridge."
The committee also said it saw no reason why the controlled archive should ever increase further, given the end of a backlog of up to 450,000 asylum cases and the end of the UKBA's review of outstanding migration cases.
Keith Vaz, the committee's chairman, said: "The UK Border Agency is still not providing the efficient, effective service that Parliament expects.
"The so-called 'controlled archive' has become a dumping ground for cases where the UK Border Agency has lost track of the applicant."
He added: "The Prime Minister himself recently called for members of the public to provide intelligence on immigrants.
"There is little point in encouraging people to do this if the border agency continues to fail to manage the intelligence it receives or to keep track of those who apply to stay.
"A fit for purpose immigration system needs to keep track of applicants rather than allowing them to go missing."
Shadow immigration minister Chris Bryant said: "These numbers betray a shocking failure at the heart of this Tory-led Government.
"The Government's chaotic approach to immigration enforcement has meant 100,000 people have been quietly dumped into the 'untraceable' file and borders staff have given up on dealing with them."
Immigration Minister Damian Green said: "The immigration system we inherited was chaotic and this Government is working to fix the mistakes of the past by making better decisions, ensuring cases are properly tracked, improving intelligence and speeding up removals.
"I am determined to deal with the historic asylum cases left by the last government and we are making real progress tackling the archive to trace these individuals.
"Since taking office we have nearly doubled the number of failed asylum seekers removed within one year of their application and now conclude cases quicker with almost two-thirds decided within a month."