Junk food firms must stop marketing their products to children - or face a total advertising ban, health chiefs have said.

The World Health Organisation (WHO) said food giants are exploiting loopholes in regulations to bombard children with adverts on YouTube and Facebook.

And one of Public Health England’s advisers called for the rules to be redrawn - with advertisements for fatty and sugary foods banned entirely if children were found to be being exposed to them.

The call came as research from the University of Liverpool found three-quarters of under 16s are being exposed to such adverts on social media - despite rules which are supposed to protect them.

A new report by the WHO today warns that watchdogs are failing to keep pace with sophisticated marketing techniques used to promote junk foods to ever younger markets.

It warns of “unequivocal evidence” that exposure to such products is fuelling a global obesity crisis.

In the UK, advertising authorities say that products high in fat and sugar can only be advertised if at least three quarters of the audience is adult.

But Nick Sheron, a clinical adviser to PHE, said these rules did not go far enough - calling for a ban on such adverts if children were found to be being exposed to them.