More than 17,000 asylum seekers whose claims have been withdrawn are missing across the UK, the Home Office has admitted.
The revelation came as MPs discussed Rishi Sunak's target to eliminate the asylum backlog by the end of the year.
Thousands of applications were discontinued by the Home Office after claimants failed to respond.
"I don't think we know where all these people are," a senior official told the home affairs select committee.
Huge numbers of migrants are still coming to Britain via small boats PA
When asked whether some people had returned to their home countries, Simon Ridley, a senior civil servant in the Home Office, said he did not know.
Claims are withdrawn if asylum seekers refuse to respond to two successive case worker interview requests or questionnaires.
This happened 17,316 times since December last year.
Any claimants who have their application withdrawn and remain in the UK they will be here illegally and can be removed.
The government has always been confident it would reach the target set by the prime minister, the most senior civil servant, Sir Matthew Rycroft said.
He added that they had hired more case workers to tackle claims.
But Conservative MP Tim Loughton, who is on the select committee, said: "Is it strange, that conveniently, when faced with a very stiff target, there has been a three-fold increase for the undetermined reasons of people magically not going forward with their claims?"
The Home Office is negotiating a treaty which Rishi Sunak says will satisfy the Supreme Court's concerns about Rwanda being a safe third country.
Lee Anderson has demanded a cap on net migration in a furious rant in the CommonsParliament TV
GB News presenter Lee Anderson demanded figures on many rejected asylum seekers had been deported in the past three years – excluding criminals and Albanians.
Ridley replied: “I don’t think we have [those numbers].” The Home Office permanent secretary then said they would “write to the committee with those numbers”.
Anderson said: “I’m sorry … but I find it absolutely staggering that the big boss hasn’t got a clue – not just on this question, but on nearly every other question we’ve asked today.”