The laptop contained unencrypted, but password protected, information about individuals who were members of six of The Pensions Trust's schemes - and held data including name, address, date of birth, national insurance numbers, name of employer, salary details and information on nominees.

For those in receipt of a pension, the data also included bank account details.

The laptop was stolen from NorthgateArinso's Marlow offices, where the laptop was being used as a database for development, training and performance testing.

NorthgateArinso supplies the software that the Pensions Trust uses for its pensions administration system.

The Pensions Trust said it had reported the theft to the Information Commissioner and The Pensions Regulator - and was in "detailed discussions" with NorthgateArinso regarding the theft.

It said the police were informed as soon as the laptop was reported missing and an investigation is ongoing - but noted the view of the police and NorthgateArinso is that this was an opportunistic theft and the laptop itself, and not the data, was the thief's target.

The Pensions Trust has written to every individual whose details were contained in the stolen laptop and to all the employers who participate in The Pensions Trust's schemes - apologising on behalf of NorthgateArinso and ourselves, explaining what has happened and outlining the protections it has put in place.

The scheme has also set up a helpline for members and employers who wish to discuss this further with The Pensions Trust

In addition, The Pensions Trust said it had appointed the services of CIFAS - The UK's Fraud Prevention Service - to protect all members whose details were contained in the stolen laptop.

The service alerts financial institutions that their data might be at risk - allowing credit card companies and banks to be extra vigilant.

Pensions Trust board of directors chairwoman Lynda Howe said: "NorthgateArinso has expressed their regret that this theft has occurred and investigations are ongoing.

"I can confirm that The Pensions Trust has now withdrawn access to personal member data from NorthgateArinso and have also instructed them to delete any existing personal member data they hold. We are hopeful that this incident will not have any impact on members but, as a precaution, we have arranged for them to be protected by CIFAS and have set up a members' helpline."

In a statement, NorthgateArinso said: "We can confirm that a laptop computer was recently stolen from a secure computer room in our Marlow offices. The laptop contained some old data belonging to The Pensions Trust, who we have kept fully informed of the situation.

"Data from other NorthgateArinso customers was not on the laptop and is not at any risk. We have no reason to believe this was a targeted data theft. We have been working closely with the police to help with their ongoing investigations."