British supermarket Morrisons has reportedly suffered a major data breach which saw the pay-roll data of a large but unspecified number of its 100,000 staff stolen and published on a website.

In an email sent to staff and later seen by TV media, the attack was said to have involved the theft of names, addresses and bank account details of workers “from all levels of the organisation."

Morrisons became aware of the posting of the data on a website – since taken down – only last night, the email said.

“Initial investigations suggest that this theft was not the result of an external penetration of our systems,” the supermarket said. "We can confirm there has been no loss of customer data and no colleague will be left financially disadvantaged."

This seems to suggest that the breach was caused by an internal hack, an extremely unusual event if that turns out to be the case. Data breaches are overwhelmingly the result of external hacking. Another unusual element of the incident is that it affects the firm's employees rather than customers.

The firm said it was working with then police to identify the source of the breach. The seriousness of the incident is underlined by the fact that chief executive Dalton Philips is to head the investigation.

The news comes at the end of a disastrous week for the firm in which it reported slumping sales and a loss a £176 million pre-tax loss.

Despite the apparent seriousness of the breach, there could be an opportunity for Morrisons to gain something positive from the incident, suggested Arbor Networks' EMEA solutions architect, Darren Anstee.

“How Morrisons is perceived to deal with this incident is key, and will likely directly affect  its reputation with both employees and the general public. Managing an incident like this efficiently can actually enhance reputation, if done well,” he said.