Children's taste buds are being altered by a bombardment of junk food adverts, the country’s chief medical officer has warned, as ministers pledge to crack down on Facebook and YouTube promotions.

Prof Dame Sally spoke out as the Government launched proposals for a 9pm watershed on advertising foods high in fat, sugar and salt, covering both television and online media.

Ministers said they would examine all options in a bid to tackle childhood obesity, which they said was now one of the biggest health problems facing Britain.

The consultation proposes that unhealthy products should not be advertised on TV screens before 9pm.

And it says restrictions on social media and online streaming site could be covered by the same ban - with advertisers only excluded if they could provide strong evidence that they are not targeting children.

Other options put forward include changing thresholds which allow foods to be advertised, or placing time limits on adverts. Currently those classed as unhealthy should not be advertised online if more than the quarter of the audience is children.