The disc - which contains surnames and initials, national insurance numbers, dates of birth and plan numbers for pension policy holders - was lost as it was transported from HMRC offices in Newcastle to Standard Life's headquarters by an external courier in late September, says HMRC in a statement.

Standard Life spokesperson Yvonne Savage told Finextra that the CD in question was one of a batch of 20 sent out by The Revenue to various firms. The other 19 reached their destinations, but the disc containing the Standard Life data failed to arrive.

Savage says the data on the CD is encrypted, but the firm has still been monitoring the affected accounts since the disc disappeared and so far there has been no suspicious activity.

HMRC says customers have been informed of the incident and "precautionary measures have been put in place to check customers' records for any fraudulent activity". Dedicated hotlines have been set up to deal with enquiries.

"We have also reviewed our arrangements and introduced safeguards to prevent this happening in future," the HMRC says.

In a separate statement Standard Life says it "immediately took all necessary security steps to protect those customers affected". The insurer says it is "confident that all appropriate action has been taken to safeguard the customers affected".

The data security incident is the second in a month involving the HMRC. In October the tax authority said a laptop had been stolen containing data on up to 2000 people with investment ISAs. According to press reports a HMRC staff member had been using the PC for a routine audit of tax information from a number of investment firms.

Earlier this year Bank of Scotland had to notify more than 60,000 mortgage customers that a computer disc containing their personal information has been lost in the postal system.