More than 40 small TV and film production companies behind shows such as Derry Girls and Say Yes to the Dress have come together to warn that the government’s proposed privatisation of Channel 4 could put them out of business.

The 44 companies, dotted across Northern Ireland, Scotland, Wales and Yorkshire, have taken out a full-page advertisement in Tuesday’s edition of the Daily Telegraph – apparently timed to hit readers during the Conservative party conference in Manchester.

The ad, placed in what is traditionally seen as the Tories’ house newspaper, argues that selling Channel 4 – which is state-owned but commercially funded through TV ad revenue and is not required to turn a profit – would be the antithesis of the government’s pledge to “level up” businesses and communities across the country.

“It would cost jobs, reduce investment, and place companies at risk in the nations and regions,” the ad states. “From production companies who should know.”

Firms that have co-signed the ad include Hat Trick, the maker of the hit comedy Derry Girls, and True North, the indie behind shows such as Say Yes to the Dress: Lancashire.

“A privatised Channel 4 would be a disaster for all small to medium-sized production companies,” said Jannine Waddell, the managing director of Waddell Media, the producer of series including Britain’s Most Expensive Houses.

“No matter what the government says, the programming remit would change and that would hurt all of us. Would a commercially focused owner really get behind something like the Paralympics? We are Channel 4’s biggest supplier in Northern Ireland, and the impact here would be massive.”

Last month the former culture secretary John Whittingdale, who led a 2016 exploration to privatise Channel 4 that was ultimately abandoned by the government, said any new owner would have to maintain the broadcaster’s commitments to regional output and commissioning shows from independent producers.

A report by Ampere Analysis found that over the past two years Channel 4 worked with more than 200 small production companies, more than any other broadcaster, with 140 of those relying on it for half or more of all their TV work. It argued that a new owner would make deep cuts to Channel 4’s programming to make it more profitable, which could put up to 60 small production companies at risk of going out of business.

“The view across the industry is that, in the drive to ‘level up’ Britain, a Channel 4 in public ownership is the best ally the government could have,” said Dermot Lavery, a co-founder of DoubleBand Films, maker of programmes including Britain’s Ancient Tracks.

He said the broadcaster “would help hundreds of new privately owned [production] companies to be seeded – the indies of the future – making sure the largesse in public service broadcasting can be spread fairly across the country. A privately owned Channel 4 would kill that.”