A pre-9pm ban on junk food TV advertising before 9pm would cost Channel 4£40m a year, the chief executive has said, suggesting advertisers would move to YouTube and Facebook which can “hyper-target” children.

Alex Mahon told MPs that if the government introduced a blanket pre-watershed ban on TV ads promoting products high in fat, salt and sugar it could backfire.

“Young people are often watching on digital platforms now, where they are hyper-targeted,” she told the digital, culture, media and sport select committee on Tuesday. “From an advertiser perspective, they might just put that [TV advertising] money into the digital platforms where that can really, really target young people. The government needs to make sure changes don’t have consequences that are worse than the current situation.”

On Monday, the government unveiled a strategy to tackle childhood obesity in the UK, which is the most overweight nation in western Europe.

The strategy includes a ban on “two for one” offers on unhealthy food and junk food at supermarket checkouts, the sale of energy drinks to children, and a consultation on whether to introduce a pre-9pm ban on junk food ads on TV.

Mahon said overall spending on UK TV of ads promoting products high in fat, salt and sugar was £200m. Channel 4 takes about £40m from advertisers promoting such products.

She said that while the £40m was only a “few percent” of Channel 4’s TV ad revenue total of more than £900m, it was “still substantial and could make a [big] impact” on the broadcaster’s ability to invest in programming.

She said a blanket ban was a blunt instrument that would catch products such as olive oil and chutney, which should not be blocked from advertising before 9pm, and would not achieve the government’s aims.

“Our concern is that if every pound previously spent on TV is switched to YouTube or Google and targeted at children I don’t think that is the intention of the obesity strategy,” she said. “The biggest change in viewing [away from TV to online] is occurring with children under-16, that is why we are concerned.”

The government’s obesity strategy also planned to consult on how to introduce “similar protection for children viewing adverts online”.

Charles Gurassa, the chair of Channel 4, said a blanket ban would affect investment in the creation of new children’s TV programmes.

Mahon also revealed an improvement in the disparity between the amount men and women are paid at the broadcaster.

Channel 4 has reduced its mean gender pay gap from 28.6% last year to 22.7% this year, which means men are still paid on average over a fifth more per hour than women.

“I made clear earlier this year that the size of Channel 4’s gender pay gap was unacceptable, especially for an organisation like ours which champions diversity and inclusion,” Mahon said. “I’m pleased that our focus on the progression of women to senior levels of the organisation has already had an impact, but there remains much more for us to do.”