British prime minister David Cameron said he would not have hired Andy Coulson as the administration's communications chief if he knew the former editor of the now-defunt News of the World (NoW) would get involved in allegations concerning phone hackings, when being questioned in parliament on Wednesday.

Cameron admitted he took the responsibility for deciding to hire Coulson and defended his decision, saying that "with hindsight" it appeared he should not have employed Coulson as head of communications.

However, he denied any fault on hiring Coulson because issues about Coulson's possible link to the phone hacking scandal didn't come up when he invited him into office, and what Coulson did during his work at the Downing 10 was undisputably appropriate.

"If it turns out Coulson knew about the phone hacking at his time at the NoW, then he has lied to the police and lied to me, and he should face severe criminal charges," Cameron added.

But Labor party leader Ed Milliband said repeated warnings about Coulson's suitability for the job as Cameron's press spokesman had been ignored, and that Cameron made wrong choice to stick with Coulson.

Cameron emphasized there still has been no evidence showing Coulson knew about phone hacking while he was editor of the NoW.

"Had I been shown evidence linking Coulson to hacking he would have been fired," Cameron said.

Meanwhile, Cameron denied he was aware Neil Wallis, a former NoW deputy editor who was later questioned by police investigating phone hackings, was giving advice to the Metropolitan Police.

The prime minister underlined the government has well-led police investigation looking at corruption in media and policing.

Cameron has earlier shortened his trip to South Africa to make an emergency statement to the parliament on the phone hacking crisis.