Andy Coulson had begun undergoing high-security vetting in November, about three months before he resigned as David Cameron's director of communications, the Guardian has learned.

Downing Street has been under pressure to explain why the former News of the World editor was not subjected to developed vetting, the high-security checking process most previous No 10 press secretaries had undergone, when

he took office. Coulson's successor, Craig Oliver, and his former deputy, Gabby Bertin, have undergone – or are undergoing – such vetting.

The Labour leader, Ed Miliband, said: "People are rightly asking questions about why that didn't happen because it did happen for his predecessors. It's happening apparently for his successor."

The prime minister's critics say he may have been content with Coulson's lower security threshold because he was concerned that Coulson might not have passed the vetting. Alternatively, Coulson himself may have suggested

the lower level of vetting.

A Whitehall source said the decision not to subject Coulson to developed vetting was taken by Jeremy Heywood, the Downing Street permanent secretary.

The source said it was decided that Coulson's role did not require access to highly secret material and developed vetting was a costly, unnecessary expense.

Coulson's lower level of clearance – categorised "security check" – did allow him "long-term, frequent and uncontrolled access" to material designated "secret" and, under supervision, to "top secret" material, the source stressed.

The source also said:

• The fact that "press secretaries with access to secret material have not always had a happy history" was a consideration in deciding to give Coulson a lower level of vetting.

• It was decided that Coulson needed developed vetting after communication problems surrounding the discovery of an explosive device on a plane at East Midlands airport in October 2010.

The vetting process can take three to six months and had not been completed when Coulson resigned in January, saying the phone-hacking scandal meant he could no longer work effectively.

The New York Times published a major investigation of the phone-hacking scandal in September 2010 but the source insisted the decision to subject Coulson to developed vetting was not connected to the report or the resulting spate of coverage in the UK media.

The source said Cameron was not informed of Coulson's initial security status but was informed when the decision was taken that he should undergo the process in November 2010.

According to the source, Coulson was not consulted on what type of vetting he would undergo, and had not received any indication of whether his developed vetting process was successful when he resigned in January.

The Cabinet Office said: "We do not comment on the vetting level or status of individual current/former members of staff."