The 'Strip That Down' singer - who recently split from Cheryl after a two-and-a-half year romance - was furious when he saw Jean-Bernard Fernandez-Versini partying with pals at the same nightclub as him in Monaco recently and he launched into a foul-mouthed tirade.
A source told The Sun: ''The whole night he was standing behind JB looking to fight him. He was stood there with his arms crossed and he kept pointing at JB - nobody could understand what he wanted.
''And JB was just ignoring him because he was like: 'I'm 35 years old, he's a 20-year-old kid. I don't want to fight this kid in Monaco - it's supposed to be a smart place'.
''In the toilets, one of JB's friends turned round to see Liam looking at him and it flared up. He said: 'What the f**k are you looking at?' and Liam said 'Listen, your friend JB is a f*****g ****.' Security stopped things escalating but as he left he looked at him and said: 'I will f**k you up.' It was all a bit embarrassing.''
JB was annoyed that Liam went after his friend and confronted the 24-year-old singer about it at the club Twiga but Liam had no response.
And restaurateur JB later told pals that Liam was only a kid who needed a hug.
This is the second time Liam and Jean-Bernard have clashed since the One Direction star split from Cheryl, 35, the mother of his 15-month-old son Bear, in July.
The pair narrowly avoided an altercation at the Chiltern Firehouse earlier this month.
A source said at the time: ''Liam was worse for wear. He was goading JB but JB said he was not going to get involved with a 'little 24 year old boy'.
''He's not one to cause a fuss and knew Liam was only acting like that because was a little tipsy.''
However, JB later lashed out at Liam, calling him a ''little man'' who was wrong for her.
JB is also believed to have told friends that he wants Cheryl back.
A source said: ''JB always knew Liam and Cheryl wouldn't last - she is 10 years older.
''If Cheryl walked back into his life he would give their relationship another go.
''Marriage has always been sacred for JB. He loved Cheryl deeply.
''She was the only family he had. He was upset they finished and always wanted to protect her and still does.''
Cheryl and Jean-Bernard married in a secret ceremony in Mustique in July 2014 after three months of dating and split up 18 months later.
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