Roy Keane has revealed he snubbed a handshake from Nottingham Forest refereeing consultant Mark Clattenburg back in 2022.
And the former Republic of Ireland captain insists it's because of something Clattenburg said 'in a book'.
Clattenburg is currently in the spotlight after Nottingham Forest controversially accused Stuart Attwell, who was on VAR duty, of being a 'Luton Town fan' following their 2-0 defeat to Everton on Sunday.
Forest felt they should have been given three penalties during the match at the City Ground.
Roy Keane has opened up on snubbing Mark Clattenburg
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And Keane, it seems, isn't Clattenburg's biggest fan.
Gary Neville, speaking on the newest episode of the Stick to Football podcast, revealed Keane snubbed the former Premier League referee by saying: "He's opened up a can of worms now!
"So basically, we're walking along this place in the World Cup, the three of us were having a chat and Mark Clattenburg walks towards us."
Keane then interjected by saying: "But he was strutting, wasn't he?"
Neville added: "Right, he's worked with us at the previous World Cups as our refereeing consultant, we trained with him and got to know him a bit.
"So basically, we're walking along and I'm like 'Hi Mark, how are you?' and then Wrighty goes 'Mark, good to see you!' and then he [Keane] hangs back a few yards.
"And then he goes over to Roy to shake his hand and Roy goes [makes aggressive handshake motion].
"No word of a lie!"
Neville continued: "He threw his hand away!
"And I'm like, 'What's going on here?! I know he's a bit feisty but...'. You [Keane] carried on walking and we were left talking to him and I said, 'Ref, is there a problem?"
When asked why he snubbed Clattenburg, Keane bluntly said: "He'd said something in a book after he worked with us."
The former midfielder, who also played for Manchester United, then criticised Forest for their VAR statement and said his old side was lacking 'class'.
"Messy. Forest at the moment going back over the last few months with the ownership, I don’t think they’ve shown any class. That’s the way I’d look at it.
"The way they were carrying on, on the pitch, who was that against, Liverpool? They were all on the pitch, the statements coming out.
"You look at the ownership. I was lucky to play for Forest - brilliant club - but I don’t like that kind of behaviour at the moment with what they’re doing.
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Roy Keane snubbed Mark Clattenburg because of something the Nottingham Forest chief wrote in a book
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"They’re entitled to come out and defend themselves, fight their own corner of course, but looking over the last few years I don’t like what I see really with the behaviour at an ownership level."
Neville, meanwhile, has accused Clattenburg of acting like a 'Navy seal' while condemning Forest for their actions.
"You've got Clattenburg almost like a Navy Seal in the director's box crawling along to the CEO (whispering)," he said.
"The reason I asked him to resign was because if he actually saw that Luton tweet that went out, he actually has to distance himself from it or he's part of it.
"If he's part of that, I'm like 'wow'.
Roy Keane and Gary Neville have ripped into Nottingham Forest refereeing consultant Mark Clattenburg
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"The second part of it is it's football, right. If you're the leader of that football club, since Mark Clattenburg came in, in the last 10 games they've won one match.
"In the 10 games before, they won four.
"You never give football players or employees a get-out excuse ever.
"He's given them an excuse to not win."