Police officers at Britain's second biggest airport made unauthorised terrorism stops and searches, the Home Office has revealed.
Terror stops: Key strategy in major public spaces
Sussex Police did not have proper approval for the special powers used at Gatwick Airport earlier this year.
A Home Office investigation found the force made 259 stops without the required ministerial backing.
Sussex Police is to apologise in writing to all those stopped at Gatwick between 3 and 25 September this year.
The error came to light on Wednesday in a written ministerial statement to Parliament and has sparked a review of all use of the power to date.
Under the special terrorism powers, known as Section 44, police can stop and search people, without having reasonable suspicion, in areas designated as a likely target for an attack.
The power was designed with major public areas in mind, such as airports, railway station and major tourist attractions.
Police chiefs authorise the power - but they must notify the Home Secretary who then has 48 hours to give retrospective approval or to countermand the order.
In the case of Sussex Police in September, that ministerial authorisation never came, said Tony McNulty, minister for counter-terrorism and security.
"In this case, due primarily to a process failure, the application was not passed to the Home Office for ministerial authorisation - but the force continued to use the powers for the period up to 25 September when the next authorisation was submitted," said Mr McNulty.
"Sussex police has confirmed that 259 stops and searches were carried out at Gatwick Airport during the unauthorised period."
Mr McNulty said none of the stops led to arrests - but Home Office security officials have also uncovered a "similar incident" involving the force in June 2003.
The error came to light when the force applied to renew the powers covering Gatwick, prompting security officials at the Home Office to investigate the Sussex's actions.
Mr McNulty said that he had expressed concern to both Sussex's Chief Constable Martin Richards - and alerted the government's independent terror powers watchdog, Lord Carlile.
Officials were also reviewing previous use of the power around the country and police chiefs had agreed "more robust" procedures.
"All steps have now been taken to ensure ... that such regrettable and serious omissions do not occur again," said Mr McNulty.
Police chiefs say the power is used strategically to create "disruption" and discourage terrorism planning, such as reconnaissance of potential targets.
Critics say it is a blunt tool that has been used disproportionately, souring critical community relations in some parts of London.