CONFIDENTIAL tapes containing the personal and medical details of thousands of staff from Hampshire firms.
The tapes contained highly sensitive information including details ranging from illnesses to home addresses.
Thieves targeted a security van while it was transporting the tapes between offices to be backed up.
The stolen data includes information on staff from a number of companies in Hampshire including finance giant Skandia.
Thousands of people have received letters telling them their details were on the stolen tapes that were owned by the company Medisure which handles healthcare cover for a number of firms in the county.
While the stolen tapes are said to contain no financial information, they did contain employee names, home addresses, age, details of dependants, details of any health care claims and scanned copies of correspondence.
Medicare insist that the data would not be easy to retrieve from the tapes to someone without specialist knowledge and that the risk of identity theft is low.
Skandia - which employs more than 1,700 people in its Southampton office - enrolled its staff on a scheme which was operated by Medisure.
One former Skandia employee, who wished to remain anonymous, said: "When you entrust important details to a respected company providing health care through your employer you expect your documents to be properly safeguarded.
"I understand from Medisure that the data was stolen by an opportunistic thief, surely a passing criminal shouldn't have the opportunity to steal my personal details because it falls into their lap."
Another said: "There may have been no financial information on the disk this time but surely that was just good luck not judgement. It is very concerning."
Lorren Wyatt, Director of HR at Skandia UK said: "Skandia operates a strict policy for data protection which insists that data is held securely.We have no reason to believe Medisure failed to meet its obligations.
"Our contract with Medisure ended four years ago and this incident is regrettable.
However, we are reassured that the risk of the data being misused is low.
"Skandia has worked closely with Medisure to ensure all those affected, including past as well as current employees, are contacted and given full information about the situation."
A Medisure spokesman said they could not say how many people's details were on the stolen tapes or what other Hampshire companies were affected.
He said: "Our primary consideration throughout has been to work with each of the clients affected. All have been advised of the situation."
Somerset and Avon police are investigating the theft which took place in Bristol in April, although letters from Medisure to those affected have only just been sent out. A spokesman added that an arrest had been made.
The Financial Services Authority and Information Commissioner were also notified of the crime.