The battle to renovate and operate a downsized Nassau Coliseum may be about to get nasty.

Bruce Ratner, one of four who is vying for the job and who is wooing Nassau County executives by promising to play six Islanders games at the 41-year-old arena even after they move to Barclays Center, may not be able to deliver on that promise, The Post has learned.

Madison Square Garden, which owns the NHL’s Rangers and is also bidding on the job, has to approve such a move — and that OK is not guaranteed, sources said.

Ratner took a full-page ad in a local newspaper yesterday touting the six-game slate, calling it “Nassau Like Never Before.”

But those plans may melt as the battle for renovation rights gets heated.

Several sources said the Islanders would need permission from both the NHL and the Rangers to play games at both arenas — and that is not a sure thing.

“You can’t run around playing games wherever you want to,” a sports owner not directly involved in the battle said.

A vote to choose a renovation developer is expected in the next two months.

Barclays CEO Brett Yormark told The Post, “The NHL is very much aware of our desire to play six games at Nassau Coliseum, and fans are excited about it, too.”

MSG and the NHL declined to comment.