Plans for the new watershed have been put out for public consultation in a bid to combat the growing crisis, the Department of Health and Social Care (DHSC) said.
One in three children leaves primary school overweight or obese and the number of children classed as seriously obese is at a record high, it added.
The news came as deputy Labour leader Tom Watson attacked fast food chain McDonald's forthcoming "Monopoly" promotion as a "grotesque marketing ploy" which he said encourages over-eating, labelling it a "danger to public health" on Twitter.
Mr Watson wrote to McDonald's UK asking for the competition, due to start on Wednesday, to be cancelled.
The letter states: "It is unacceptable that this campaign aims to manipulate families into ordering junk food more frequently and in bigger portions, in the faint hope of winning a holiday, a car, or a cash prize many would otherwise struggle to afford", the Observer reported.
Campaigners, doctors and politicians welcomed the announcement about the proposed advertising ban.
TV chef Jamie Oliver said: "If we don't find effective ways to improve our kids' health, UK children will live shorter lives than their parents.
"It's a fact that kids are hugely influenced by junk food ads - so the media and the food industry has a real opportunity here to do something about it."
Adverts for foods high in fat, sugar and salt will be consulted on, with the proposed pre-9pm ban affecting TV programmes, online streaming sites and social media companies, the DHSC said.
Junk food adverts during children's TV shows have been banned since 2007 but research by broadcasting regulator Ofcom claimed youngsters spend 64% of their TV viewing time watching shows not aimed specifically at them.
Public Health Minister Steve Brine said: "The NHS is already preparing to treat more and more children for the serious effects of extreme obesity in the future, so we have a duty to address the underlying causes because we believe passionately in our NHS."
Up to 1,000 more children per year are expected to require treatment for severe obesity-related problems like diabetes and asthma by 2022-23, the DHSC said.
It has previously called the current situation a "rising epidemic in childhood obesity".
Professor Russell Viner, president of the Royal College of Paediatrics and Child Health, said he wanted the watershed "implemented as soon as possible".
Caroline Cerny, of the Obesity Health Alliance said: "The evidence is clear - junk food adverts are impacting children's health and the current regulations are outdated and riddled with loopholes."
The proposed ban will not affect adverts for staple foods like butter, olive oil or meat.
Cancer Research UK is among bodies calling for the new watershed. Watching one extra junk food advert a week, beyond the average of six, leads to children eating an extra 18,000 calories a year, the charity said last year.
The study estimated the additional calories are the equivalent of around 70 Mars bars or 60 cheeseburgers - and could amount to a 5lb (2.3kg) weight gain annually.
A McDonald's UK spokesman said: "This year's Monopoly campaign sees customers receive prize labels on carrot bags, salads and our Big Flavour Wraps range and we have removed the incentive to 'go large', providing the same number of prize labels and chances to win on a medium meal as you get on a large.
"Nutrition information is clearly displayed online, on our app, in restaurant and across our packaging and we continue to review, refine and reformulate our menu to reduce saturated fat, salt and sugar."