The health secretary has said he is "looking very seriously" at making vaccinations compulsory for all children going to school in England. Some experts have suggested it may be necessary to address falling rates of immunisation and a surge in diseases like measles.
Matt Hancock told an event at the Tory conference he had taken legal advice this week on how to go about it. Unvaccinated children were "putting other children at risk", he said.
The uptake of the MMR (Measles, Mumps and Rubella) vaccine in particular has been declining in many countries.
The reason is not clear. Rates dipped in the 1990s following publication of a report linking MMR to autism, but partly recovered after that research was discredited and disproved.
However, the volume of anti-vaccine sentiment on social media has been swelling and in March, the head of NHS England warned "vaccination deniers" were gaining traction online.
The health secretary has said before he was willing to look at "all options" to boost England's vaccination levels, including compulsory immunisation - and while he did not want to "reach the point" of imposing jabs, he would "rule nothing out".
Mr Hancock appeared to firm up his stance at the conference in Manchester on Sunday.
Measles is highly infectious and can cause serious health complications, including damaging the lungs and brain.
There were more than 82,500 cases in Europe in 2018 - the highest number in a decade and three times the total reported in 2017.
In England, the proportion of children receiving both doses of the MMR jab by their fifth birthday has fallen over the last four years to 87.2%. This is below the 95% said to provide "herd immunity", the level considered by experts to protect a population from a disease.