Ian Edmondson
Ian Edmondson originally pleaded not guilty to phone-hacking chargesGetty

Former News Of The World news editor Ian Edmondson has pleaded guilty to phone hacking while working at the now-defunct paper.

He originally denied conspiring to hack phones at the Old Bailey 16 months ago, at the start of the phone-hacking trial, which is one of the longest and most expensive trials in British legal history.

Edmondson, 45, stood trial with the rest of the defendants, including the paper's former editor and ex-Conservative Party director of communications, Andy Coulson, and News International chief executive Rebekah Brooks, but was ruled "unfit" to continue in December 2013.

After a hearing in July declared he was fit to stand trial, Edmondson pleaded guilty to conspiring with private detective Glenn Mulcaire to intercept private voicemails between October 2000 and August 2006.

During the trial, it emerged nearly a quarter of all the phones Mulcaire hacked came from Edmondson's instructions, including former Deputy Prime Minister Lord Prescott, Sir Paul McCartney and actors Jude Law and Sienna Miller.

Earlier this year, Coulson was sentenced to 18 months in jail after being found guilty at the Old Bailey. Former NotW chief correspondent Neville Thurlbeck and news editor Greg Miskiw were both handed six-month sentences after admitting phone-hacking charges, while Mulcaire, assistant news editor James Weatherup and former reporter Dan Evans all received suspended sentences.

Brooks and the rest of the defendants were cleared of all charges.

Edmondson is expected to receive a custodial sentence following his guilty plea.