Hollywood actress Jessica Biel is courting controversy after seemingly coming out against childhood vaccinations.

The former 7th Heaven star and wife to singer Justin Timberlake appeared at the California State Assembly this week alongside prominent anti-vaccination activist Robert Kennedy Jr, the pair joining forces to lobby against a bill that would limit exemptions from vaccinations.

Biel appeared on Kennedy Jr's Instagram as he applauded her for her campaigning work: "Please say thank you to the courageous @jessicabiel for a busy and productive day at the California State House," he wrote.

The Daily Beast reports that Biel and Kennedy were lobbying against SB 276, a state bill that "limits medical exemptions from vaccinations without a public health official's prior approval".

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Speaking to that outlet, Kennedy confirmed Biel had joined him, but refuted the term "anti-vaxx", which he deemed ""pharmaceutical propaganda", news.com.au reports.

"I would say that she was for safe vaccines and for medical freedom," he said.

Kennedy told The Daily Beast that if passed, the bill would force doctors to vaccinate children even if the children had been determined to be "too fragile to receive vaccinations".

He said that Biel "knows what she's talking about … She's upset about this issue because of its particular cruelty. She has friends who have been vaccine-injured who would be forced to leave the state."

News that Biel is campaigning against mandatory childhood vaccinations may not come as a complete surprise — in 2015, US outlet InTouch Weekly reported that Biel and Timberlake would not be vaccinating their son Silas Randall Timberlake:

"Jessica is refusing to vaccinate him. She feels that vaccination could cause complications. I'm sure Jessica believes that she's making the right decision, but hopefully she and Justin will do some more research on this and change their minds," the outlet quoted a source as saying.

While Biel has today remained publicly silent on the issue, those on social media have been quick to slam her: