Andy Coulson resigned Friday, saying he was unable to dedicate enough time to the job because of the "drip drip" about activities at the News of the World while he was editor, The Daily Telegraph reported.
Coulson had previously resigned from his position at the tabloid newspaper because of the alleged phone-hacking scandal, the Telegraph said.
The newspaper's former royal editor, Clive Goodman, was jailed in 2007 for conspiracy to access voice mails left for royal aides, and private investigator Glenn Mulcaire was jailed on the same charge.
"Unfortunately, continued coverage of events connected to my old job at the News of the World has made it difficult for me to give the 110 per cent needed in this role," Coulson said.
Cameron said he is "very sorry" Coulson felt "compelled" to resign because of continuing pressure over the phone-tapping allegations.