Blake Shelton is suddenly looking much slimmer — and pals dish the country heavyweight ditched his unhealthy diet because fed-up wife Gwen Stefani banned him from their bed until he lost some weight.

Insiders exclusively tell Life & Style the “God’s Country” singer, 47, dropped as much as 20 pounds by eating lean proteins, veggies and fruits — and even cut back on beer after his fit duet partner laid down the law. “Blake feels like a new man,” a source says. “Before, he was huffing and puffing going up stairs — but now he’s darting up and down with no trouble at all!”

The tipster adds that the No Doubt frontwoman, 54, got her hubby to replace his favorite frozen pizzas and other junk foods with sensible, well-portioned meals.

“She said no nookie until she saw improvement. It may have sounded harsh, but she had to make Blake realize she was serious!” the insider spills. “He’s tried to lose weight before with little to no success. But Gwen put her foot down. She wants him to stick around, and his unhealthy habits were grating on her nerves!”

The source admits Blake initially considered his new eating plan “torture” — but explains as the fat melted away, Ol’ Red got his mojo back. The insider confides, “Their love life has greatly improved, and Blake has more stamina and energy now than he’s had in years. He realizes he really let himself go. Now there’s sizzle in their marriage again.”