Council staff are expected to contact a helpline after a laptop was stolen containing the national insurance and bank details of thousands of employees.
Worcestershire County Council employs 18,000 people but will not say how many are affected. It is feared they are now at risk of identity fraud.
The laptop, in possession of the council's IT suppliers, was stolen in a street robbery two weeks ago.
Staff were told on Friday and some said they were unhappy at the delay.
One staff member told BBC News he was horrified such a mistake had been allowed to occur.
The workers were informed by letter which stated: "The personal details include names and addresses.
"Other details relating to National Insurance and Bank Accounts are also held on the computer but are not so easily deciphered.
"Although the theft appears random rather than planned and the computer is password protected there is nevertheless a risk that this information might be misused."
A council spokesman said the laptop was owned by Serco Solutions.