LOS ANGELES - Robert Downey Jr has warned fans not to fall for “scam artists” who claim to be him on social media and ask for money.

The actor wrote on Facebook that “one or more scam artists have taken to impersonating me online and asking some of you via private message or chat for ‘donations’ for various ‘causes’”.

The Iron Man star continued: “If you encounter someone on a chat platform claiming to be me, they are lying.”

Hundreds of fans replied, many of whom have been approached by impostors. Some said they had also been contacted by people claiming to be other stars like Ryan Reynolds, Ryan Gosling and Hugh Jackman - but the vast majority said they had quickly sniffed out the deception.

Downey Jr’s message added: “I will never ever communicate via private chat platforms, and I would never ask individual fans for money for any reason.

“Any communication from me will originate from my public, verified social media pages, and all fundraising is done via broad, public campaigns.

“I am constantly humbled by the incredible love, support, and generosity of our little community. Some want to take advantage of that, so let’s have each other’s backs.”

Fans responded with their own tales of being approached by fake Robert Downey Jrs and people impersonating other celebrities - and how they got their own back.

Earlier this year, country singer Brad Paisley warned fans about a similar scam.

Replying to a question about whether a request for cash was genuine, he wrote: “Come on people. Don’t fall for this... Don’t send money to anybody claiming to be me. Or anyone else either.”