The secret to the butt that spawned a Facebook fan page is Pilates.

Pippa Middleton says the exercises are a “weekly necessity,” according to a new report.

Middleton, whose behind drew legions of fans after she appeared in a body-hugging gown at the royal wedding, attends Pilates on the Go in London, the Daily Mail reported.

“As someone who is always ‘on the go,’ Pilates has been a wonderful escape from my busy lifestyle in London,” Middleton wrote in a testimonial on the studio’s website.

‘Whether it’s 7a.m. in the morning or 7 p.m. at night I always leave feeling calm, refreshed and invigorated,” she wrote.