Jessica Biel sparked controversy on Tuesday as she joined politician Robert F. Kennedy Jr. to lobby against a Californian vaccine bill.

The 65-year-old politician and activist, who is known for his stance against vaccinations, shared a series of pictures of himself and Jessica on his Instagram page on Tuesday, writing: "Please say thank you to the courageous @jessicabiel for a busy and productive day at the California State House."

Fellow politician Devon Mathis, currently serving in the California State Assembly, also shared a snap with Jessica online, posting on Facebook: "It was great seeing Jessica Biel this afternoon in the Capitol. It’s awesome to have amazing partners and advocates for parental rights like her! Keep the fight alive. #VoteNoOnSB276 #CaDeservesBetter."

The SB 276 bill, which was launched in March by California senator and pediatrician Dr. Richard Pan, would limit medical exemptions from vaccinations without approval from a state public health officer. It's estimated that, if the bill is officially introduced, it would cut the amount of people exempt from vaccinations by around 40 per cent.

However, Kennedy told The Daily Beast that he and Jessica share the same belief that it's not right to force children to have vaccinations if it's against their wishes.

"The biggest problem with the bill, which is something I think Jessica is concerned with, is that a doctor who has made a determination - if he has found children in this state whose doctors have determined that they’re too fragile to receive vaccinations - this bill would overrule the doctors and force them to be vaccinated anyways,” he said.

Calling The Sinner actress "extremely well-informed", Kennedy added that she's “upset about this issue because of its particular cruelty” and noted that "she has friends who have been vaccine-injured who would be forced to leave the state."

While Jessica has never publicly spoken about her stance on vaccinations, it was reported back in November 2015 that she and husband Justin Timberlake had decided not vaccinate their son Silas, now four.