Coulson is at the heart of a long-running scandal centred on Rupert Murdoch's British newspaper group and its close links with politicians.

The issue of Coulson's security vetting, one of many sub-plots in the complex saga, has raised questions about Cameron's judgment.

Testifying at the Leveson inquiry into press standards, former cabinet secretary Gus O'Donnell said Coulson had received the basic SC security clearance when he entered Downing Street rather than the more thorough DV vetting because he was not expected to be involved in security-related matters.

These arcane details have become politically sensitive because in his testimony at Leveson last week, Coulson said he "may have" had unsupervised access to top-secret documents and had attended meetings of the national security council.

Critics say this is the latest in a long list of errors of judgment by Cameron, who relied heavily on Coulson to guide his media strategy and improve his understanding of the average voter.

Coulson resigned as editor of News of the World in 2007 after one of its reporters was jailed for eavesdropping on voicemails of members of the royal household.

A few months later, Coulson was hired as spokesman for Cameron, then in opposition. When Cameron became prime minister in May 2010, Coulson was appointed his director of communications.

He stepped down in January last year under pressure over allegations that phone-hacking at the News of the World had been widespread on his watch.

Coulson has denied knowledge of criminal activities at the paper, which was shut down in July after it emerged reporters had hacked into a murdered schoolgirl's voicemails.

Cameron's critics say he should have asked more questions about the phone-hacking affair before hiring Coulson.

O'Donnell said SC vetting had been appropriate. He said the aim of DV vetting was to determine "whether you're blackmailable in terms of your financial position or your personal life" and therefore could pose a security risk. This was not relevant in Coulson's case.

O'Donnell said DV vetting would not have involved a closer look at events at the News of the World. "It wouldn't have gone into enormous detail about phone-hacking," he said.

This did not placate the Guardian newspaper, which broke the phone-hacking story.

"O'Donnell says DV process would not have looked into hacking - only seeks to establish 'if you're blackmailable'. Err, contradiction?" wrote the paper's deputy editor, Ian Katz.