UPDATE: Greenslade has since written the following:

Let me try to cast a little light on the story about MPs from the Commons media select committee being put under surveillance by the News of the World.

This appears to have gained legs because of an interview I gave to a US site, Media Matters, on Saturday afternoon.

By chance, I had been previously speaking to a source who told me that every member had been shadowed for a period of three days.

Due to the context of the conversation, I took him to mean early in 2011. However, it transpired - after I had given the interview - that he was talking about mid-2009.

That is a very important difference, of course, but I cleared up the misunderstanding with a couple of tweets and also in a phone call to one of the committee members.

However, the substantive matter of MPs being followed remains a live issue - which they are planning to discuss - and this is what I understand to have happened.

While discussing the News Corp. hacking scandal during an appearance on SiriusXM's Media Matters Radio on Saturday, Guardian writer Roy Greenslade said that a confidential source told him that “for three days, only six, seven months ago, every single member of the parliamentary committee investigating this matter were followed by private eyes and/or members of staff of the newspaper. And only after some of the staff protested that they didn't want to do it did the -- were they called off.”

Previous reports had revealed that celebrities, politicians, a teenage crime victim, and police had been victims of phone hacking by The News of the World, the News Corp. tabloid that was shut down as a result of the scandal.

From the November 12 edition of SiriusXM's Media Matters Radio:

ARI RABIN-HAVT (co-host): Roy, do you think that News Corp. is a criminal enterprise? To be pointed about it.

GREENSLADE: I think, you know, in truth, it would be way over the top for me to say that. And I was equally distressed that Tom Watson had referred to the Mafia. But, you know, one can understand why he did, for goodness' sake. When we talk about omerta, we talk about a code of silence. Well, for several years, News International, which is the U.K. division of News Corporation, has maintained a code of silence. They have lied, they have deceived, and they have briefed private eyes to follow people. And in Tom Watson's case, you can understand why he was incredibly angry to find that he's been under secret surveillance for several occasions.

And I can add a little to what Steven said, because I do have a source who was in The News of the World, the newspaper that was closed after this debacle. And he tells me that for three days, only six, seven months ago, every single member of the parliamentary committee investigating this matter were followed by private eyes and/or members of staff of the newspaper. And only after some of the staff protested that they didn't want to do it did the -- were they called off.

Now, that goes even beyond what we've previously heard. And I've only just been told this in the last 24 hours, so your audience is getting a scoop here.

RABIN-HAVT: So, are we breaking a little news here?

GREENSLADE: You are breaking a story. I wish I could name the source, but, of course, I have it confidentially.