Channel 4 is to sell its London headquarters as deep job cuts that are expected to particularly affect staff based in the capital result in the search for new, smaller premises.

The broadcaster, which has been based in the £90m headquarters in Horseferry Road, Victoria, since 1994, intends to find a smaller location in central London in the next few years as it moves to employ most of its staff outside London.

Channel 4 confirmed plans to reduce staff numbers by 240, its biggest round of layoffs in more than 15 years, as part of an accelerated shift to streaming during the worst downturn in TV advertising since 2008.

The broadcaster, which employs more than 1,200 staff, said it intended to cut 200 jobs, as revealed by the Guardian three weeks ago.

Channel 4 said it would also close 40 unfilled roles, making an overall 18% reduction in headcount, as the broadcaster retrenched after undergoing a rapid expansion that included staff numbers hitting record levels.

The latest restructuring is likely to put significant pressure on London-based staff again, given Channel 4’s promise to increase employee numbers in the “nations and regions” to 600 by 2025. The broadcaster’s “national headquarters” are in Leeds.

“With 600 roles based outside London by the end of 2025, lower headcount in London overall, and a shift to flexible working, Channel 4 will find a new fit-for-purpose office space in central London,” the broadcaster said.

The cuts form part of a five-year strategy, called Fast Forward, that aims to shift Channel 4 away from a dependency on traditional TV advertising to digital income streams.

Alex Mahon, the chief executive of Channel 4, said: “As we shift our centre of gravity from linear to digital, our proposals will focus cost reductions on legacy activity. It does involve making difficult decisions. I am very sad that some of our excellent colleagues will lose their jobs because of the changes ahead.”

The broadcaster said about 70% of the unfilled roles it was closing came from its legacy operations, and that the overall cuts programme would return the number of employees close to 2021 levels.

Channel 4 said it aimed to “divest from legacy operations to support digital priorities” and create a “leaner, simpler and nimbler” business to protect the broadcaster’s “long-term sustainability”.

It added that it intended to shut “small linear channels” that no longer delivered “revenues or public value at scale”, starting with the Box TV channels this year, followed by “others at the right time”.

In 2022, two-thirds of Channel 4’s total £1.14bn revenues came from traditional TV advertising. The broadcaster said on Monday that by 2030 it aimed to reach a “tipping point”, with at least 50% of total revenues coming from digital sources.

Mahon outlined a diversification plan that included owning and exploiting Channel 4’s own shows for the first time, creating an e-commerce business worth “double digit millions” annually by 2030 and doubling the number of members to its ad-free streaming tier Channel4+, which costs £3.99 a month, by the end of the decade.

“Channel 4 was designed to be ahead of the curve and has never stood still,” Mahon said. “The rate of change in our market is only speeding up. Our new strategy will accelerate our digital transformation.”

Last year, Mahon told the Commons culture committee that the state of the TV ad market was so bad it was in “shock territory”. She added that the broadcaster expected to make losses in each of the next two years, after three years of surpluses.