Google says a small number of its enterprise customers mistakenly had their passwords stored on its systems in plaintext.

If you have a Google account, Google's core sign-in system is designed not to know your password.

The search giant disclosed the exposure Tuesday but declined to say exactly how many enterprise customers were affected. “We recently notified a subset of our enterprise G Suite customers that some passwords were stored in our encrypted internal systems unhashed,” said Google vice president of engineering Suzanne Frey.

The company said that only G Suite enterprise customers were impacted, but not regular Gmail accounts.

The tech giant said it had notified G Suite administrators to change the impacted passwords.

Google on Wednesday extended an apology to its G Suite customers.

"We apologise to our users and will do better," she added.

Most G Suite customers are companies that signed-up for enterprise versions of Gmail, Google Docs, Google Sites, Google Drive, and Google's various other services.

No consumer Gmail accounts were affected by the security lapse, said Frey.

Storing passwords without cryptographic hashes expose them to hacking risk as they become readable.

Passwords are typically scrambled using a hashing algorithm to prevent them from being read by humans. G Suite administrators are able to manually upload, set and recover new user passwords for company users, which helps in situations where new employees are on-boarded. But Google said it discovered in April that the way it implemented password setting and recovery for its enterprise offering in 2005 was faulty and improperly stored a copy of the password in plaintext.

Google has since removed the feature.

Google said the bug at the heart of this security breach was an old tool it developed back in the 2000s.

"The tool (located in the admin console) allowed administrators to upload or manually set user passwords for their company's users," the company said today.