The 91-year-old actress, best known for playing Dot Branning on BBC soap EastEnders, was taking part in new health programme 100 Years Younger in 21 Days.
Along with seven other celebrities, June was staying at the Rejuvenation Clinic in Sardinia.
Claire King, Roy Walker, Sid Owen, Sherrie Hewson, Shaun Ryder, Russell Grant and Sandra Martin were left stunned by flexible June’s antics.
During tonight’s first episode of the series, all the celebrities underwent therapies, including a coffee enema for some, and a snail facial for others.
Afterwards, June decided to show off her Tibetan yoga, which she believes keeps her fit, despite smoking for 72 years.
Lying on the floor, June was seen to lift her legs up off the floor, but ended up flashing her underwear to the group.
June explained: “You get onto the floor and you lay on your back.
“And then you lift those! Excuse me draws! Then you bring this head up towards those legs when they’re up.”
As her co-stars shrieked and tried to cover her modesty, they realised that it may not have been an accident after all.
Shaun laughed: “I thought I should jump up and whip my t-shirt off and put it over her and then it clicked and I thought, you know what? She knows exactly what she’s doing. She’s game for anything!”
Sid chuckled: “June is just a happy soul which keeps you young, I suppose.”
June added: “The only thing that keeps you alive is to laugh and not to take things too much to heart.”
Viewers instantly took to social media following the broadcast.
One viewer tweeted: “OMG the amazing June Brown in #100YearsYoungerin21Days She is absolutely incredible, I just love her!”
“#JuneBrown in #100YearsYoungerin21Days 90 years old, legs in the air with a skirt on, ooh mind me drawers lolololol she is a f**n ledge!!!! @bbceastenders,” posted another.
A third added: “#100YearsYoungerin21Days June Brown! What a legend!!!! Still got it lady Always be young at heart.”
“June Brown is the star of the show #100YearsYoungerin21Days,” wrote another.
100 Years Younger in 21 Days continues next Tuesday on ITV at 9pm.