BLAKE SHELTON is openly lusting for his “Voice” co-star Christina Aguilera, and it’s sparked the biggest dustup so far with his fed-up wife Miranda Lambert, a source says.

Blake has made no secret of his attraction to Christina. He even admitted last year that before he wed Miranda in 2011, the country spitfire had given him a “free pass” to hook up with the Latina sexpot.

And now Christina has returned to join Blake on the coaches’ panel after a season’s hiatus – looking hotter than ever following a 30-pound weight loss – prompting horndog Blake to make an inappropriate remark that sent Miranda into a jealous rage, the sources add.

“Blake’s already confessed to having a crush on Christina, and Miranda is furious that he won’t stop talking about her,” said a source. “After he came home from the ‘Voice’ set raving about how fabulous Christina looks, Miranda lit into him.

“She was really upset, especially after Blake gushed about how sexy Christina looked and how he wished Miranda had as good a figure.

“Miranda told him in no uncertain terms that the ‘free pass’ she gave him before is now officially and permanently revoked.”

Blake and Miranda’s two-year marriage has been rocked by cheating allegations be­fore – and it doesn’t help that they’re often apart due to their hectic schedules. As we recently re­ported, Miranda, 29, warned her 37-year-old hubby to stay away from Cher after he hand-picked the “Woman’s World” singer to serve as his team mentor during the Battle Rounds on the hit NBC show.

Miranda previously confronted singer Cady Groves, 24, over suspicions she was hav­ing an affair with Blake. And the “Mama’s Broken Heart” singer also cautioned Blake to stay away from former “Voice” winner Cassadee Pope, 24, whom he mentored on the show.

Now, sources say Miranda feels threatened by Christina, 32, even though the blonde bombshell reportedly dropped the weight to get in shape for her upcoming wedding to former production assistant Matthew Rutler.

Meanwhile, Miranda “has packed on” pounds over the past year, mostly due to mari­tal stress,” divulged the source. “She feels down on herself and worries she’s not hot enough to satisfy her husband.

“With Christina looking so sexy these days, Miranda is a bundle of nerves. Her mar­riage is hanging by a thread, and Miranda fears the temptation of being around Chris­tina for long hours day after day may be too much for Blake.

“She knows better than anyone that Blake doesn’t have the best impulse control.”

Blake and Christina’s flirtatious behavior first raised eyebrows when the two re­portedly began drinking together in their dressing rooms, sparking whispers among “Voice” staffers that their relationship was too close for comfort.

Around the same time, Blake confessed to having the hots for Christina – and said his wife had given him one “free pass” to have a fling with the “Your Body” singer before they were married.

“At this point,” added the source, “Miranda is eating her words over that ‘free pass’ comment.”