Half of advertisements children see on television are for junk food, sugary drinks and outlets such as McDonald’s, prompting fresh calls for tougher action to limit exposure to them.

Research by the Institute for Fiscal Studies has found that 50% of all TV advertisements seen by children aged between four and 15 are for products high in fat, salt or sugar.

The revelation comes amid calls for the government to impose much tougher restrictions on the ability of food manufacturers and retailers to advertise junk foods as part of a crackdown on childhood obesity.

The advertising of such foods during children’s programmes has been banned since 2007. But research by the broadcasting regulator Ofcom has shown that children spend 64% of their TV viewing time watching shows not aimed specifically at them.

Health, medical and children’s organisations want ministers to introduce a tougher regime as part of the second phase of their childhood obesity strategy, due later this year.

An IFS briefing paper on children’s exposure to food and drink advertising on the small screen says: “50% of the TV advertising for food and drinks that children saw in 2015 was for products that are HFSS [high in fat, salt or sugar] or for restaurants and bars. [Of that] 39% was for products that were HFSS [and] 11% was for restaurants and bars, the majority of which was for fast food chains. (Over half of this was for McDonald’s).”

Significantly, 70% of the ads children see for HFSS products go out before the 9pm TV viewing watershed. Health campaigners are urging the government to ban all HFSS advertising before that time, because of the large numbers of under-18s who watch programmes in the early evening.

“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,” said Rebekah Stroud, a research economist at the IFS and co-author of the briefing.

Ads for HFSS products shown in breaks during family entertainment shows such as ITV’s The X Factor have attracted particular concern because children make up a large proportion of their audience.

“This report demonstrates in stark fashion the critical urgency of tackling our nationwide childhood obesity crisis,” said Sharon Hodgson MP, Labour’s public health minister.

“The prime minister must deliver substantial action in this year’s revised childhood obesity strategy, and stop her government’s reckless cuts to local authority and education budgets.”

The findings come days after NHS figures showed that 22,646 children aged 10 and 11 in England in school year six – one in 25 of that year group – are severely obese. That means they have been found to have a body mass index of at least 40.

A Department of Health and Social Care spokesperson said ministers were preparing further action on childhood obesity. “Our sugar tax is funding school sports programmes and nutritious breakfasts for the poorest children, and we’re investing in further research into the links between obesity and inequality.

“We’ve always said that our 2016 plan was the start of the conversation, not the final word on obesity. We are in the process of working up an updated plan, and will be in a position to say more shortly.”