Glasgow City Council has been fined £150,000 by the Information Commissioner after two unprotected laptops containing the personal details of tens of thousands of council tax payers were stolen from its offices.

The breach of the Data Protection Act, one of the largest in Scots history, came after Glasgow City Council failed to heed repeated warnings from the Information Commissioner's Office (ICO) to ensure all computers containing people's details were encrypted.

One of the stolen laptops contained the council's creditor payment history file, including the personal information of more than 20,000 people and the bank account details of 6069 individuals.

During the ICO investigation it also emerged a further 74 laptops have gone missing, and council officers have been unable to say what was stored on their hard drives.

Ken Macdonald, the ICO's Assistant Commissioner for Scotland, said the loss of so much private information beggared belief and condemned the council for its "flagrant disregard" for security.

The council was already operating under an enforcement notice issued in 2009 after the loss of a memory stick containing the details of sex offenders, their victims and witnesses.

On that occasion, Glasgow City Council was only rebuked by the ICO because it did not have the power to hand out fines for breaches.

In the latest incident, the two unencrypted laptops were stolen from the council's offices on May 28 last year while an area was being refurbished.

Despite complaints from staff of theft and a lack of security, one device had been locked away in a storage drawer with the key placed in another drawer where the second laptop was kept.

The second drawer was subsequently left unlocked overnight, allowing the thief easy access to both computers.

Mr Macdonald said: "How an organisation can fail to notice that 74 unencrypted laptops have gone missing beggars belief. The fact that these laptops have never been recovered, and no record was made of the information stored on them, means that we will probably never know the true extent of this breach, or how many people's details have been compromised."

On top of the fine, the ICO has also served the local authority with an enforcement notice requiring it to carry out a full audit of any Information Technology assets used to process personal data, and to arrange for all of its managers to receive computer asset management training.

A Glasgow City Council spokesman said: "This data loss should not have happened and we took immediate steps to ensure it does not happen again.

"It is important to note that the number of unencrypted laptops was already coming down when this theft occurred."