The ad urges parents of children with autism to consider going vegan to “alleviate the devastating effects of autism”.

“More research is needed, but scientific studies have shown that many autistic kids improve dramatically when put on a diet free of dairy foods”.

Vicki Gibbs, a clinical psychologist at Australia's largest non-profit, autism service provider, Autism Spectrum Australia told Fairfax Media the strength of PETA’s statement was concerning.

“Parents of kids with autism are a pretty vulnerable group,” Ms Gibbs said.

“Putting something up as such a strong warning could give them a lot of unnecessary guilt and anxiety, as many may not be educated enough to tell the difference between fact and fiction.”

Peta cites two scientific studies back up its billboard: one with just 20 participants and one with 36.

Both studies were discredited in an independent review of 14 studies into the link between autism and casein (proteins commonly found in milk) by the University of Texas in 2010.

The ad caused an uproar when PETA launched the ad a few years ago resulting in the US billboard company that was hosting the posters to cancel the partnership.

But the ad has resurfaced recently and PETA hasn’t retracted it, with the information page detailing the alleged link still available on its website.

The group has history of a publishing ads engineered to court controversy in their bid to get noticed, including an ad comparing meat eating to the Holocaust, which was banned by the Germany’s High Court and numerous ads comparing women to meat.