Jennifer Garner’s boyfriend, John Miller, issued an ultimatum after Ben Affleck was spotted getting cozy with his ex-wife, a source exclusively tells Page Six.

The CaliBurger CEO “has always been supportive of Ben and Jen’s co-parenting relationship but feels Ben crossed the line,” the insider says.

“John knows there’s nothing going on between Jen and Ben, but he doesn’t think those photos are a good look and feels it’s disrespectful to their relationship.”

Jennifer Garner’s boyfriend, John Miller, issued an ultimatum after Ben Affleck was spotted getting cozy with his ex-wife, a source exclusively tells Page Six. BACKGRID

The CaliBurger CEO “has always been supportive of Ben and Jen’s co-parenting relationship but feels Ben crossed the line,” the insider says. ZapatA/MEGA

“John knows there’s nothing going on between Jen and Ben, but he doesn’t think those photos are a good look and feels it’s disrespectful to their relationship,” the source explains. BACKGRID

According to the source, Miller, 47, “gave Jen an ultimatum” — “he doesn’t want to see anything like that again or he has no choice but to walk away.”

The “Gone Girl” star, 52, was seen affectionately grabbing the “13 Going on 30” actress, also 52, as the couple celebrated their son Samuel’s 13th birthday on March 2.

The Oscar winner grabbed the Emmy nominee around the waist as she focused her paintball gun on a target at the Combat Paintball Park in Castaic, Calif.

Miller’s proposal comes after the “Gone Girl” star was seen affectionately grabbing the “13 Going on 30” actress as the couple celebrated their son Samuel’s 13th birthday on March 2. GJC / BACKGRID

According to the source, Miller “gave Jen an ultimatum” — “he doesn’t want to see anything like that again or he has no choice but to walk away.” Felipe Ramales / SplashNews.com

The Oscar winner grabbed the Emmy nominee around the waist as she focused her paintball gun on a target at the Combat Paintball Park in Castaic, Calif. GJC / BACKGRID

Affleck pulled Garner in toward him, and she appeared to welcome the embrace.

The “Daredevil” co-stars — who also share Violet, 19, and Seraphina, 16 — looked cozy as they laughed and closely chatted while participating in the festivities.

Reps for Miller, Garner and Affleck did not immediately respond to Page Six’s requests for comment.

The “Daredevil” co-stars looked cozy as they laughed and closely chatted while participating in the festivities. BACKGRID

The exes were married from 2005 to 2015. BACKGRID

As Page Six previously reported, Miller hasn’t been “thrilled” about the exes — who were married from 2005 to 2018 — spending more time together.

Miller “knows Ben and Jen have been leaning on each other more than ever lately and have a really close relationship.”

“Their growing bond is getting to be a bit much when Ben spends all the holidays with them like Christmas and Thanksgiving,” the insider said. “John feels like a third wheel.”