Arnold Schwarzenegger has been rumbled after he went undercover as physical fitness instructor Howard as part of a fundraising stunt.

The Hollywood star donned a moustache, fake ponytail and cap to raise money for after-school clubs, Metro reported.

He told one gym goer: "Stop being a baby, this is not a baby gym," and gave over-the-top instructions to another on exactly how to drink water from a communal fountain.

Arnie also pretended to be a receptionist b ut with his distinctive voice it wasn't long before The Terminator actor was recognised, the newspaper said.

The actor was raising cash for After-School All-Stars, an American organisation that provides free mentorship, homework help, tutoring and activities to school pupils from low-income families.