MIRANDA LAMBERT has launched a secret plan to save her marriage to her heartthrob hubby Blake Shelton!

The 29-year-old firecracker has already confronted a young singer she suspected of cheating with Blake, and insiders say she now has enlisted “spies” to keep tabs on her handsome spouse.

But friends fear it will be im­possible to tame country singer Blake, who’s become a crossover superstar since signing on to be a coach on the hit NBC sing­ing competition “The Voice.” Blake has denied that he has cheated but told a network morning show he loves the rumors because it means he’s popular.

“Miranda knows her marriage is just one cheating night away from imploding,” revealed a close source. “Any girl that comes sniffin’ around her husband will have to answer to her in all her fury.”

Budding pop singer/songwriter Cady Groves, 23, found out what Miranda’s wrath feels like when she touched off a cheating scandal with Blake.

The rising star enjoyed a flirtatious Twitter relationship with Blake for nearly two years, and he even appeared in Cady’s debut music video, ironically named “This Little Girl.”

Alarmed, Miranda flew into action.

“She felt like this young gal was making a fool of her and a mockery of her marriage to Blake,” disclosed an insider.

She phoned the “other” woman and let her have it in a shocking showdown.

“Miranda told her point-blank to stay away from her husband if she knew what was good for her,” divulged the source.

“Cady sounded shocked, denied everything and com­plimented Miranda, telling her how much she admired her. But Miranda just cut her off, saying she’s dealt with girls like her before and she better stop it – like yesterday.”

A shaken Cady quickly called Blake and asked: “What should I do?” said the close source.

“Blake told her, ‘If you value your life, I’d stop tweeting me.’”

Although Cady and Blake, 36, both claim their relationship is innocent, others say they’ve been hooking up for the past two years – and a miffed Miranda has report­edly called her lanky husband “just a big ol’ horndog.”

She believes Blake minds his p’s and q’s while taping his show in Los Angeles, as sources say she’s told pals: “At least he’s in one place and I’ve got spies on him out there.”

But her biggest fear is the months they will spend apart touring the country.

“The Voice” has its season finale in mid-June and then Blake hits the road. Miranda will be touring as well. They can go weeks without seeing each other, and Miranda’s suspicions creep in, said the close source.

Her fears are nothing new. In­siders reveal she had doubts about Blake’s fidelity from the begin­ning. In fact, although Miranda denies it, there have long been rumors that Blake’s relationship with Miranda started with a fling while he was married to his high school sweetheart Kaynette Wil­liams. Blake and Miranda ended up dating for six years before she finally “took a leap of faith” and got hitched.

Sadly, as The ENQUIRER re­vealed last month, friends don’t give the couple much of a chance to make it. Confided one pal: “They are two ships passing in the night, drifting further and further apart.”

Agreed the close source: “Mi­randa is facing an uphill battle. I just don’t think their marriage is going to survive.”