The 23-year-old, who plays for Manchester City, posted a picture of himself training with his England team-mates at St George's Park, with a tattoo of an assault rifle on his right calf clearly visible.
Lucy Cope, who founded Mothers Against Guns after her son Damian was shot dead outside a club in central London in July 2002, said the ink was "totally unacceptable".
She told the Sun: "We demand he has the tattoo lasered off or covered up with a different tattoo.
"If he refuses he should be dropped from the England team.
"He's supposed to be a role model but chooses to glamorise guns."
But the former Liverpool player, who is among five England strikers heading to Russia for the World Cup, alluded to the tattoo having a "deeper meaning".
In a post on Instagram, he said: "When I was 2 my father died from being gunned down to death I made a promise to myself I would never touch a gun in my life time, I shoot with my right foot so it has a deeper meaning N still unfinished."