Thousands of air passengers have bypassed border controls after they were sent the wrong way upon arrival in the UK, new figures reveal.
Carriers and airport operators are responsible for directing all travellers on international commercial flights towards immigration desks for passport and security checks.
But data obtained by the Press Association shows that from 2013 to 2017, more than 11,000 arrivals were not presented to Border Force immediately after landing.
A "misdirection" is logged when a passenger reaches a part of the airport situated beyond border controls without first having been cleared for entry by immigration officers.
Figures released by the Home Office following a Freedom of Information request show 2,328 passengers were misdirected last year - an average of six a day, and an increase of 70% compared with 1,364 in 2016.
Border Force recorded 2,394, 2,665, and 2,278 misdirected passengers in 2013, 2014 and 2015 respectively - giving a total of 11,029 over the five years.
While the FOI response did not detail the circumstances of the incidents, a Government factsheet published in 2015 stated that misdirections usually occur because the wrong doors have been opened at the arrival gate, or because the airline or airport operator sent the passenger to the wrong place.
Conservative MP Tim Loughton, a member of the Commons Home Affairs Committee, said: "There are enough concerns about the capacity of Border Force to cope with the increasing demands of incoming visitors even before Brexit without learning that thousands of passengers are getting in through the side door unchecked.
"Airports are responsible for security and absolutely need to face consequences when they fail to ensure that proper checks are being made to ensure we know who is coming into our country."
The figures were revealed as ministers consider whether to press ahead with plans to impose fines of up to £50,000 on airlines and airports if they fail to take all reasonable steps to ensure arriving passengers are correctly directed to border controls.
The Immigration Act 2016 provides a legal basis for Border Force to levy a penalty if a misdirection occurs.
Under the plans, fines could range from £2,500 to £50,000 depending on factors such as the number of passengers involved and action taken by the airport operator or carrier after the incident.
But the Airport Operators Association (AOA) argues the proposal is "disproportionate".
A spokesman for the AOA described border security as a "top priority" for airports.
He said: "The fact that the number of misdirected passengers as a proportion of total passengers travelling through UK airports has fallen significantly since 2013 demonstrate that our efforts to reduce the number of misdirections are having an effect.
"We are committed to working with airlines, ground handlers and Border Force to continue to improve on our track record.
"We do not believe that the proposed civil penalty should be part of that ongoing work as it is disproportionate in light of the numbers of passengers involved and our continued commitment to work collaboratively to reduce numbers even further."
The Home Office stressed that all misdirected passengers are subsequently returned to border control or, where this is not possible, retrospectively checked against security and immigration watchlists.
There are no examples of dangerous individuals arriving unchecked because of a misdirection, the department added.
A Home Office spokesman said: "The security of our border is paramount -- which is why 100% of scheduled passengers are checked when arriving in the UK.
"While the overwhelming majority of arriving passengers are properly presented at border control, there are still a relatively small but unacceptable number of cases each year where passengers are misdirected.
"These people are identified and the necessary checks carried out, but this adds an unnecessary administrative burden.
"We are determined to eradicate these errors and believe a civil penalty is a vital tool in ensuring this happens.
"We are currently considering the contributions to the consultation on the proposed introduction of such a penalty and continue to work with airlines and airport operators on all areas of security."