A computer data disk containing personal details of around 2,000 members of British Council staff has been lost.
The loss, involving names, national insurance numbers, salary and bank account details of the Council's UK staff, is the latest in a string of cases of official information going astray in recent months.
But the British Council, which is a Foreign Office-sponsored arms-length organisation devoted to promoting knowledge of British culture and the English language overseas, said that the missing disk was securely encrypted to keep its contents safe if it falls into the wrong hands.
A British Council spokesman said that the disk, containing some details which appear on staff payslips, was lost while being transported from the organisation's payroll data supplier to its human resources department by courier firm TNT on a routine monthly delivery in December.
"It was sent according to our agreed process with the usual secure TNT courier service but was not received by our human resources team. TNT has informed us that they are still taking steps to find the disk," said the spokesman.
He added: "The data only included staff records and no information about external British Council contacts is involved.
"The data on the disk was compressed using a proprietary algorithm; furthermore it is not an ordinary CD-ROM or DVD, but an optical disk that can only be read by a particular type of reader with a specific version of specialist software. This software is no longer manufactured and cannot be purchased.
"These precautions ensure that the data is extremely secure in the unlikely event that the disk should fall into the wrong hands. The system for transferring data is being reviewed and in the meantime the data is not being sent."
Liberal Democrat home affairs spokesman Chris Huhne, who uncovered the incident, said it became public a day ahead of the second reading in the House of Commons of the Coroners and Justice Bill, which will give Government departments more power to share data.
Mr Huhne said: "Despite the Prime Minister's assurances after the last data loss on a memory stick in a pub car park, it is clear that Whitehall's culture of carelessness about confidential data has not ended."
