Zac Efron went litter picking in London over the weekend

An onlooker told the Daily Mirror newspaper: "He really mucked in. There was no fanfare over his arrival or anything, he just turned up and got involved. Half the volunteers didn't know who he was, but the kids were really excited."

It's not known why the former 'High School Musical' star decided to volunteer but he is currently filming an environmental documentary on food for Netflix.

Zac started taking interest in his food earlier this year when he decided to go vegan - a plant-based diet that eliminates meat, dairy and any byproducts of animals - and he's already noticed such a huge improvement in his health.

He said recently: "Just like everyone else, I wake up, look in the mirror, and I see the imperfections and the bags under my eyes, and think, 'Oh, man, is this really the best I've ever looked? I'm getting older? What the heck?'

"Recently what I've started doing is searching for the cure inside that's causing the symptoms of what I'm getting on the outside. Getting more sleep has really taken away the bags under my eyes, so I don't need to worry about it as much. There are other lifestyle changes - I've been experimenting with eating purely vegan. That's completely changed the way that my body works, and the way that I metabolise food, the way it turns into energy, the way that I sleep. It's been brilliant. It's been great for my exercise, and great for my routine."