A person from Afghanistan on the UK's no-fly list has been flown into Birmingham as part of the British evacuation operation in a potential security breach, MPs have been told.
The individual, who is not being identified by the government, is understood to have reached UK soil on a British military plane, with officials being alerted "overnight".
The "no-fly list" is designed to block individuals who are considered a security threat from reaching the UK.
The government has looked at the case and decided "they are not a person of interest" and no further action will be taken, so the person is free to go.
A Home Office spokesperson said: "An individual was flagged to the Home Office as part of the rigorous checks process, involving the police, security services and others.
However, upon further investigation, they are not a person of interest to the security agencies or law enforcement."
Tonight's statement by the Home Office suggests big problems with the no fly list system since they let a flagged individual on the list enter the UK - then apparently later concluded that he should not have been blocked from coming in.
In a sense of the challenge facing British officials as they mount an urgent evacuation effort, the number of "hits" on the list over the past week is more than would normally be expected in a year.
But it is still only a tiny handful compared with the thousands of Afghans the UK has flown to safety from the Taliban, including former interpreters and other staff who risked their lives to support the British military and diplomatic mission in Afghanistan.
If any assessment had found that the person did pose a threat, the UK government would not have been able to return them to Afghanistan after Britain suspended flights.
The news of the Birmingham case emerged at a briefing for MPs by ministers and officials.
Opposition MPs worried it could have been a security breach, although government sources said the fact the individual had been identified showed the list is working. They also stressed that people can be flagged on the "no-fly" list for a wide variety of reasons.
British officials at the airport in Kabul are warning of a spike in impersonations, forged documents and forged passports.
Ministers are saying this is why we cannot "open the gates" to allow anyone in without checks.
Sky News understands that the US does not have any such checks, and people will be processed in the US on arrival.
MPs were told today that five people on Britain's "no-fly list" attempted to leave Afghanistan with UK help. Four of the five were prevented from flying, but one person made it through to Birmingham.
The revelation was made by a senior Border Force official in a call to MPs earlier today. He also said that one more individual was intercepted in Frankfurt, and he was blocked from continuing his journey to the UK though his family were allowed to continue.
This is a significant increase in the number of people on the "no-fly" list trying to gain access to the UK.
Normally from Afghanistan, officials would get one attempt every four or five months - so five red flags in a week is a significant increase.
Kevin Foster, the Home Office minister, told MPs on the call that while the UK must help those under threat from the Taliban, they must be "mindful" of people who pose a threat to the UK.
Ministers are warning against diluting or speeding up checks to get desperate Afghans out of the country as the Defence Secretary Ben Wallace warns the operation on the ground could be over in "hours now, not days".
A government spokesperson said: "There are people in Afghanistan who represent a serious threat to national security and public safety.
"That is why thorough checks are taking place by government, our world-class intelligence agencies and others.
"If someone is assessed as presenting as a risk to our country, we will take action."
British officials said there is a real risk of Islamic State militants seeking to exploit the UK's emergency scheme designed to help former Afghan interpreters and other local staff who worked for the UK military and diplomats in Afghanistan over the past 20 years.
A huge effort is under way to run security checks on each application, but there is also an urgency to get families on to aircraft before Kabul airport's evacuation mission is forced to stop - something that could happen imminently.
There is also a concern about Islamic State trying to target British troops conducting the evacuation by inserting a suicide bomber into the chaotic crowds massing outside the airport.
The longer the evacuation continues, the greater the risk, security sources said.