The Information Commissioner’s Office (ICO) said it was notified of the cyber incident by Marriott in November 2018.

It said a variety of personal data contained in approximately 339 million guest records globally were exposed by the incident. Seven million related to UK residents.

Marriott said an internal investigation had shown that a hacker had been able to access the guest reservation database of its Starwood division in the US since 2014, potentially including passport and credit card numbers.

Marriott acquired Starwood in 2016, but the exposure of customer information was not discovered until 2018.

The ICO’s investigation found that Marriott failed to undertake sufficient due diligence when it bought Starwood and should also have done more to secure its systems.

Information Commissioner Elizabeth Denham said:“The GDPR makes it clear that organisations must be accountable for the personal data they hold. This can include carrying out proper due diligence when making a corporate acquisition, and putting in place proper accountability measures to assess not only what personal data has been acquired, but also how it is protected.

“Personal data has a real value so organisations have a legal duty to ensure its security, just like they would do with any other asset. If that doesn’t happen, we will not hesitate to take strong action when necessary to protect the rights of the public.”

Marriott said it will appeal the fine.

Arne Sorenson, president and chief executive of Marriott International, said:  “We are disappointed with this notice of intent from the ICO, which we will contest.  Marriott has been cooperating with the ICO throughout its investigation into the incident, which involved a criminal attack against the Starwood guest reservation database.

“We deeply regret this incident happened.  We take the privacy and security of guest information very seriously and continue to work hard to meet the standard of excellence that our guests expect from Marriott.”

The Starwood guest reservation database that was attacked is no longer used for business operations.