Hundreds of people could be at risk of identity fraud after a laptop holding sensitive information was stolen from an HM Revenue and Customs worker.
Five firms had customer data on the stolen laptop
The laptop was stolen from the boot of an HMRC car last month.
A staff member had been using the PC for a routine audit of tax information from several investment firms.
HMRC has apologised for the breach, but reassured the public that data on the PC had been stored securely. Police are now investigating the incident.
An HMRC spokesman said the security breach was entirely its own fault.
"We deeply regret the incident and it's entirely down to us," he said.
The spokesman added that information was stored on the laptop using "a complex password and top level encryption".
"It is well nigh impossible for a thief to make use of the information," he said.
The theft also involved a printout of some names and financial details.
However one woman, whose details including her passport number and address were on the print out, contacted the BBC Moneybox programme to say she was not reassured.
"This is highly sensitive material and shows a total disregard by HMRC for the nature of the information with which they are entrusted by client customers," she said.
HMRC refused to comment on how many individuals may now be at risk, or how many financial institutions have had their data stolen as well.
But inquiries by the BBC suggest the computer held data on around 400 customers with high value individual savings accounts (ISAs), at each of five different companies - including Standard Life and Liontrust.
The HMRC has contacted the firms involved and asked them to contact their customers in turn.
Nigel Legge, chief executive of Liontrust, said it had taken several measures to alert and protect its customers and their investments were completely secure.
"We have now added to those stringent procedures, so that it is extremely difficult for anyone to get into those investment accounts and steal money," he said.
"And we do have identity fraud insurance so that investments clients have with us are completely safe," he added.
The theft took place in London on the night of the 20/21 September during a routine audit, which involves visiting investment companies to check the figures they have supplied about their customers' accounts.
It was reported immediately and the member of staff is now the subject of an internal investigation, as keeping the laptop in the car was an apparent breach of the HMRC's rules.