“Friends” star Matt LeBlanc is acting downright UNfriendly on the set of the show that’s made him a household name, charges an insider.

The 36-year-old star is already focusing on “Joey” — next season’s spin-off of his current comedy hit — at the expense of “Friends.”

And Matt’s enthusiasm for his own project has started to create cracks in the relationships of the closest — and most successful — ensemble cast in TV history.

“Matt is very happy about his new show, but his attitude is beginning to affect the rest of the cast — and he’s not even aware of it,” revealed the insider.

“They are all supportive of Matt and hoping his show will be a big hit, but right now they feel like he’s taking it too far when they have a whole season to finish first.

“Matt often arrives on the set announcing, ‘Joey is here!’ The first time he did that it was kind of funny and everyone clapped and laughed at him — but by the tenth time no one said anything. Most people just turned away from him and rolled their eyes.

“The cast and crew are just getting into their last season, and they need everyone focused on making this the best one yet.”

But Matt’s attention is clearly elsewhere as he discusses with the “Friends” producer — who will also be producing his spin-off — how to make “Joey” a big hit.

Disclosed the insider: “Matt regularly says things like, ‘When I’m the only one on TV next season, we’re going to do things a little differently.’

And what makes all of this so uncomfortable is that his good friend and co-star David Schwimmer is hurt that no one has offered him a show, too.

“Don’t misunderstand, David is really happy for Matt but feels he is just as talented. Sometimes he doesn’t know how to act toward Matt,” said the insider.

“Matt — who heard David was disappointed — has tried to make him feel better by offering him guest spots on his new show. But that only made David feel worse.

“No one wants to rain on Matt’s parade, but they are getting more and more impatient with his attitude. Enough already!

“They are hoping Matt’s excitement subsides so they can really get down to business and make this season one to remember.

“It would be a shame if after almost 10 years together as ‘Friends’ they all end up as enemies.”