Country music superstar Blake Shelton concluded his 23-season journey on NBC's The Voice by taking a dig at his former co-coach Adam Levine, intensifying the old rivalry between the two, according to insiders.

Levine, the 44-year-old Maroon 5 frontman, came back to the singing competition alongside his band to perform their new song "Middle Ground" on Shelton's farewell episode, four years after Levine's departure from the reality show.

However, 46-year-old Shelton, best known for his hit "Ol’ Red," was not shy about sharing his opinion.

Shelton quipped, “I’m glad Adam is singing and not speaking because every time he speaks, he says really stupid stuff.”

His criticism of Levine didn't stop there, as he labeled the tattoo-clad pop star “hands down the worst coach of anything I’ve ever seen in my life!”

“Adam felt it was a cheap shot — especially as he made the special trip to bid farewell to Blake,” shared an insider.

The two music heavyweights have a track record of trading barbs, with Shelton frequently targeting the "Moves Like Jagger" singer.

"Blake presents it as humor, but he genuinely doesn't have a high opinion of Adam," said the source.

Shelton reportedly perceives Levine as "arrogant" and believes he fails to measure up to The Voice's other coaches. The insider added, "Blake always felt Adam was more intent on drawing attention to himself than focusing on the contestants. That attitude doesn't gel with Blake."

Now, the insider intimates, "The tension that characterized their interactions on the show is at an all-time high!"

National Enquirer reported on the insider's comments.