Newcastle manager Alan Pardew has told his players they could be sued for breach of contract if they criticise the club on social networking sites.
Legal warnings have been issued after Joey Barton and Jose Enrique questioned the club's transfer policy on Twitter.
"The problem I have with Twitter is that people are twittering in an emotional state," said Pardew.
"Putting something out instantly can be very, very, damaging - not only to a football club but to any organisation."
After a friendly defeat by Leeds last Sunday, Barton was transfer-listed by Newcastle after posting comments about the management of the club.
The 28-year-old, who has more than 250,000 followers on Twitter, has continued to comment about his situation on the site throughout the week.
Pardew revealed that he consulted with Sir Alex Ferguson as the Manchester United manager has also experienced problems with players using social networking sites.
Pardew added: "It's in breach of contract if you are criticising the owner, the masseur, a player, anyone at the football club. Giving out injury news causes problems too.
"I spoke to Sir Alex Ferguson, and United's policy is that nobody at their club can comment about the football club [on Twitter]. We've notified the players this morning.
"They've probably seen it as another sort of dictatorial moment from us but we've had to issue them legally with a letter to say this is not right, it's a breach of contract and you have to understand you're going to be fined and disciplined for that. It can't happen.
"Sir Alex shared with me quite ferociously how difficult this Twitter issue is for clubs, particularly managers."