The files, which related to investments worth millions of pounds, included cheques and other sensitive data. The files also reportedly included details belonging to three lottery winners.

According to reports, the documents were found by a vehicle recovery driver on a roundabout near Reading in Berkshire. It is thought the box was being taken by a DHL courier from a Prudential building in Reading to a secure storage unit in Essex.

Reports say the box of files was handed back to Prudential. A spokesman for the UK insurer told reporters that all 200 customers are being notified of the incident.

A source close to an investigation into the incident also told reporters that Prudential has suspended all use of DHL's courier services until the investigation is completed.

The incident comes just weeks after the UK Information Commissioner's Office (ICO), which is responsible for enforcing the Data Protection Act, called on organisations to implement stronger safeguards to help protect personal data in the wake of the security breach at HM Revenue and Customs (HMRC).

The ICO called for companies and individuals to step up the fight against ID fraud following the HMRC breach, where two computers disks containing information on 25 million child benefit claimants have gone missing.

Details of the HMRC breach followed disclosure of another incident where a CD containing confidential data for around 15,000 Standard Life customers went missing while in transit from HMRC to the insurer's headquarters in Edinburgh.

The disc - which contained 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.