The company, which operates Manchester Airport, London Stansted and East Midlands Airport, said "at no point has passenger safety or aviation security been compromised" during the incident and neither MAG nor the system hacked held customers' bank or payment details.
It is understood that the majority of the data accessed was restricted to customer email addresses and related to wifi sign-ups within the airports' terminals.
Phone numbers, vehicle registration numbers and postcodes were also accessed during the hack, the company said, while operations at all three airports remained "unaffected".
A MAG spokesperson said on Thursday: "Manchester Airports group has been subject to a cyber security incident by an unauthorised third party.
"A quantity of customer data has been obtained that relates to car park, lounge and fast track bookings and in-airport wifi sign-ups at Manchester, Stansted and East Midlands airports.
"We immediately contained the risk and have been working with specialist advisers and taking appropriate steps to protect our customers and systems.
"We have informed and are working with the relevant authorities. At no point has passenger safety or aviation security been compromised."
Beverley Griffiths, senior lecturer in security and resilience at Buckinghamshire New University, said: "While email addresses, phone numbers, postcodes and vehicle registration details may appear less sensitive than financial data, large volumes of this information can be highly valuable to cyber criminals.
"Combined, these details can be used to build profiles of individuals and support targeted phishing, social engineering and fraud attempts.
"Manchester Airports Group appears to have responded quickly by containing the incident and working with the relevant authorities.
"Customers should remain vigilant for suspicious emails, texts or phone calls and verify any requests for personal information through official channels."
More than 32 million passengers flew into or out of Manchester Airport last year, while more than 30 million travelled through Stansted, in Essex, and four million through East Midlands in Leicestershire.
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