The woman was arrested on suspicion of fraud at around 8am on Thursday and was being held in custody at a west London police station that evening.
At least 300 hundred staff at the airport have had their security passes suspended while the Metropolitan Police looks into reports of a scam that surfaced on Thursday, July 28.
The Met say there is no suggestion that the alleged fraud is terrorist related.
Two women from Surrey, a 24-year-old and a 20-year-old were arrested on suspicion of fraud and money laundering on Wednesday, August 3 and have since been bailed to return to a west London police station in November.
They were arrested in connection with an allegation that money was fraudulently taken from a bank account.
Police also confirmed on Thursday evening that the 24-year-old is also under investigation in relation to the vetting process for security passes.
Hundreds of staff employed by Sodexo, which supplies cleaners and caterers to the airport, can no longer use their security passes while police investigate.