The arrest of two women at Heathrow Airport over alleged fraud has sparked a police investigation into airside passes.
The pair, who are aged 20 and 24 and from Surrey, were held on suspicion of money laundering and fraud on August 3 after money was taken from a bank account.
It is understood that the fraud investigation then triggered an inquiry into airside passes.
Scotland Yard stressed that there is no counter-terrorism link and that there is no evidence that staff airside passes have been sold.
A Metropolitan Police spokesman said: "Officers from the Met's Aviation Policing Command are investigating an allegation of fraud, reported to them on July 28.
"Detectives arrested two women from Surrey, aged 24 and 20, on suspicion of fraud and money laundering on August 3.
"They have been arrested in relation to an allegation that money has been fraudulently taken from a bank account. Other matters have come to light during the inquiries and form part of the ongoing investigation."
The bank account involved belongs to a private individual rather than a business.
Scotland Yard said the women have been released on bail until late November.
A spokesman for Heathrow Airport said: "Our top priority is the security and safety of our passengers and colleagues and we have taken appropriate action until the investigation is completed."