Who to believe? On one side we have Andy Coulson, a former showbiz hack and red-top editor, who says he knew nothing about illegal phone-hacking taking place at the News of the World during the five years he was in charge. On the other is Sean Hoare, another former showbiz hack, who was showbiz editor during Coulson's reign and who alleges that Coulson knew all along what was going on. Not only did Coulson know, says Hoare, he actively encouraged his staff to intercept the phone calls of celebrities in the pursuit of exclusives.

Spokesmen for Coulson and the NoW have attempted to discredit Hoare's fresh allegations by portraying him as an unreliable witness, a disgruntled ex-employee with drink and drugs problems. It is true that Hoare was dismissed from the paper and that he has overcome addictions. So why should he be believed?

First, because he has nothing to gain and everything to lose by admitting that he took part in illegal activities: he now risks prosecution, though the Metropolitan Police may be distracted reinvestigating the reinvestigation of the investigation, as Sir Humphrey might have said.

He has received no money, so what is his motive? Is it political? Or just revenge? Hoare says no to both. He says that justice has not been done. The NoW allowed Clive Goodman, its former Royal correspondent, to take the blame and go to prison when it was proved the paper had been hacking into the phones of Princes William and Harry. And Hoare is dismayed at the "selective amnesia" shown by Coulson and other News International executives who gave evidence to last summer's Select Committee looking into the allegations.

I am a lawyer working for clients whose phones have been hacked. But the only hard evidence lawyers had was what the police found in their raids in 2006 on Goodman and on private investigator Glenn Mulcaire. I wanted to meet someone who could throw some light on what had been going on from the inside. Sean is a rough diamond, confident but not arrogant, sensitive but not vulnerable. He has described to me the tense atmosphere of the newsroom, and the many meetings with Coulson where, he said, he played him recordings of intercepted voicemails.

The rumour is that there were two departments at the paper who hacked telephones: the newsdesk, who used Mulcaire to do the dirty work, and the features desk, which did its own. It's said that once you know how to intercept a voicemail it's easy. So easy in fact, that there's no point not doing it. The NoW denies these allegations and it is easy to dismiss them as far-fetched. But then again, organised criminality at such a high level should be, because, if true, it's as outrageous as the best tabloid splash.

Sean confirmed the rumours, and added that there were complaints from celebrity agents at the time, who suspected that the only way such sensational information could be obtained was through listening to voicemail messages. But if they dared question a story's provenance, they were simply told, "We got it from a source." Sean told me that lawyers at the NoW knew nothing of the hacking when it was going on, and would have come down on him like a ton of bricks had they known. This is not a practice one would expect a legal department to sign off.

So what to make of Sean? Is he a vital witness who has spoken out bravely about a time in the not-so-distant past when illegal activity was widespread at the NoW? Or is he just making it up? I know what I believe.