Diplomats and civil servants are to be warned about the danger of putting details of their family and career on social networking websites. The advice comes after the wife of Sir John Sawers, the next head of MI6, put family details on Facebook — which is accessible to millions of internet users.

Lady Sawers disclosed details such as the location of the London flat used by the couple and the whereabouts of their three children and of Sir John’s parents. She put no privacy protection on her account, allowing any of Facebook’s 200 million users in the open-access London network to see the entries.

Lady Sawers’ half-brother, Hugo Haig-Thomas, a former diplomat, was among those featured in family photographs on Facebook. Mr HaigThomas was an associate and researcher for David Irving, the controversial historian who was jailed in Austria in 2006 after pleading guilty to Holocaust denial.

Patrick Mercer, the Conservative chairman of the Commons counter-terrorism sub-committee, said that the entries were a serious error and potentially damaging.

“Sir John Sawers is in a very sensitive position and by revealing this sort of material his family have left him open to criticism and blackmail,” he told The Times. “We can’t have the head of MI6 being compromised by having personal details of his life being posted on Facebook.

“As a long-serving diplomat and ambassador, his family have been involved in his line of business for decades. I would have hoped they would have been much more sensitive to potential security compromises like this.”

He said that terrorists and hostile intelligence services could trawl through Facebook and Twitter websites to uncover personal details of diplomats and civil servants who might end up in highly sensitive jobs. All members of the Armed Forces are warned about Facebook and other social networking sights. Although they are not banned from the website, they have been told not to include details that could compromise their security. The same warning has been issued at MI5, MI6 and GCHQ, the Government’s eavesdropping centre in Cheltenham. Personnel will be reminded of it in a new circular this week in the wake of Lady Sawers’ Facebook entries.

David Miliband, the Foreign Secretary, was dismissive yesterday of the security implications of the incident. Speaking on The Andrew Marr Show on BBC One, he said that it was “no state secret” that Sir John wore Speedo swimming trunks on family holidays, referring to one of the published photographs. “For goodness’ sake, let’s grow up,” he said.

He described Sir John as an “outstanding professional” and denied the episode would compromise his career.

The Foreign and Commonwealth Office would not comment on whether the Sawers would have to move out of their London flat. Lady Sawers’s entries — now removed — would not have been considered a security risk had her husband followed a strictly Foreign Office path. However, Sir John started his career serving with MI6 in Yemen before switching to frontline diplomatic work, leading to his current post as Britain’s Permanent Representative at the United Nations in New York.

Sir John is not the first intelligence chief to fall victim to unwelcome publicity. When Alex Allan, former Permanent Secretary at the Ministry of Justice, was announced as the next chairman of the Cabinet Office Joint Intelligence Committee in November 2007, it was discovered that he had a personal website on which there were photographs of him on holiday and windsurfing down the River Thames complete with pinstripe suit, bowler hat, briefcase and brolly.

Dame Stella Rimington was once photographed shopping near her home in London after she had been appointed Director-General of MI5 in 1991. She was advised to leave her family home for her personal safety and moved to a new address.

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