The wife of the new head of Britain's spy agency posted pictures of her husband, family and friends on Internet networking site Facebook, prompting astonishment among security experts and calls for an enquiry.
Sir John Sawers was appointed last month to take over as head of the Secret Intelligence Service in November. The agency, popularly known as M16, has emerged from the shadows in recent years but its employees are still bound by strict secrecy rules.
In what the Mail on Sunday called an "extraordinary lapse", the new spy chief's wife, Lady Shelley Sawers, posted family pictures and details of where they live and take their holidays and who their friends and relatives are.
The details could be viewed by any of the many millions of Facebook users around the world, but were swiftly removed once authorities were alerted by the newspaper's enquiries.
Foreign Secretary David Miliband made light of the incident. He denied there had been any security breach and gave the incoming spy chief his full support.
"It's not a state secret that he wears speedo swimming trunks. For goodness' sake, let's grow up!" Miliband told the BBC.
Security experts were aghast.
"It is a most distressing and unfortunate security lapse that will take a great deal of money to put right," said Professor Anthony Glees, director of Buckingham University Centre for Security and Intelligence Studies.
Glees said the Sawers family would almost certainly need to be re-housed and the children may require extra protection.
The incident was the latest in a string of security blunders, lapses and leaks by officials that have embarrassed the government of embattled Prime Minister Gordon Brown. The timing, less than two weeks after Brown launched Britain's first national cyber security strategy, was particularly inopportune.
The Mail on Sunday, publishing the story on its front page and the pictures on a double-page spread, said the information "could potentially be useful to hostile foreign powers or terrorists".
Edward Davy, the Liberal Democrat foreign affairs spokesman, called on the government to launch an inquiry into the matter and question whether Sawers was up to the job of running Britain's oversees spying operations.
"Normally I would welcome greater openness in government for officials or politicians, but this type of exposure verges on the reckless," he said.