Around half of television food and drink adverts seen by children are for products high in fat, salt or sugar (HFSS) or for fast food restaurants, researchers have found.
The advertising of junk food products has been banned during children's programmes since 2007 but the Institute for Fiscal Studies (IFS) found that 70% of TV campaigns for HFSS products or restaurants and bars are screened prior to the 9pm watershed.
Evidence from Ofcom suggests that in 2016 children spent 64% of their viewing time watching television outside children's programming.
Some 87% of the TV advertising for food and drink seen by children aged four to 15 in 2015 was during non-children's programming, according to the IFS.
They found that 39% of these ads seen by children were for HFSS food and drink and 11% were for restaurants and bars, the majority of which were fast food outlets.
Recent discussion ahead of the second wave of the Government's childhood obesity strategy has included calls from health campaigners and political leaders to extend current restrictions on the advertising of HFSS products to cover all pre-watershed scheduling.
A new report from a leading group of MPs also endorsed calls for a 9pm watershed on junk food advertising.
The Health and Social Care Select Committee made a raft of recommendations to help solve the nation's childhood obesity epidemic, including stricter restrictions on advertising for food and drinks high in fat, sugar and salt.
Rebekah Stroud, a research economist at IFS and co-author of the briefing note, said: "Current restrictions ban the advertising of food and drink high in fat, salt and sugar during children's programming. However, a lot of the TV advertising that children see is not during children's programming, leading to calls to extend the ban to all advertising before the watershed.
"In 2015, up to 35% of the TV adverts for food and drink that children saw would have been directly affected had restrictions applied before the watershed."
Advertising Association chief executive Stephen Woodford said: "Ofcom and the Government have described UK advertising rules as already 'among the strictest in the world' and further restrictions on the advertising of HFSS food and drink would be at odds with research that shows obesity among young people varies significantly across the UK, correlating strongly to areas with increased deprivation.
"This suggests that effective action must be targeted at local level and that blanket nationwide restrictions across media are not the answer.
"Such bans will not be effective solutions, but will have an adverse impact on commercial media and have an additional bearing on the quality of media, content and jobs."