The employees who have had their passes suspended work for Sodexo, which supplies airline lounge services at the transport hub.
A spokesperson for the company 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.”
The pass suspensions come as police are investigating alleged fraud at Heathrow Airport.
Metropolitan Police detectives arrested two women from Surrey, aged 24 and 20, on suspicion of fraud and money laundering last week.
"We can confirm that officers from the Met's Aviation Policing Command are investigating an allegation of fraud," said a spokesperson for the Metropolitan Police.
"Detectives arrested two women from Surrey, aged 24 and 20, on suspicion of fraud and money laundering on Wednesday, 3 August. 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 enquiries and form part of the ongoing investigation.
"The women were taken into custody at a west London police station and have subsquently been bailed to return to the police station in late November. Detectives searched a residential address in Surrey under the Police And Criminal Evidence Act.
"Enquiries are ongoing."
Today a spokesperson at Heathrow said she couldn'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," she added.