Heathrow Airport has suspended airside passes for around 300 Sodexo employees while Scotland Yard carries out investigations into alleged fraud.
The catering company said the move is a "precautionary measure" following a suspected security pass scam.
Scotland Yard said it would not reveal details of its suspicion around airside passes, but it is not believed to involve illegal sale or cloning.
Security passes are thought to be issued to many of the 70,000 workers employed on the airport's airside.
Permanent and temporary passes are given to flight crews, shop and restaurant staff, baggage handlers, hangar and ramp workers, cargo and security employees.
Sodexo provides airline lounge services at Heathrow Airport and the suspension is thought to have mainly affected caterers and cleaning staff.
Two women from Surrey aged 20 and 24, thought to be airport workers, were arrested on 3 August on suspicion of fraud and money laundering. They were taken into custody at a west London police station and have subsequently been bailed until November.
The 24-year-old is now also being investigated in relation to the matters raised about the vetting process for security passes, the Metropolitan Police confirmed.
Police said a 28-year-old woman from Ealing was also arrested on suspicion of fraud yesterday (11 August).
Detectives from the Met's Aviation Policing Command said the arrest of the third woman related to fraudulent activity in the vetting process for airside security passes at Heathrow Airport.
The woman is currently in custody at a west London police station.
A spokesperson for Sodexo said: "We can confirm that, as a precautionary measure, the airside passes of approximately 300 employees were suspended but as this is part of an on-going investigation we are unable to comment further."
A statement from the Met said: "The Met is aware that a number of security passes held by Sodexo employees have been suspended as a precaution.
"The investigation is not terrorism-related.
"Enquiries are ongoing. We are not discussing details of this ongoing investigation further."
A spokesperson for Heathrow added: "We can't comment on the specifics of an ongoing police investigation.
"Our top priority is the security and safety of our passengers and colleagues and we have taken appropriate action until the investigation is completed."
The matter came to police attention following an allegation of fraud reported on 28 July.
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