The film director Ken Loach yesterday warned that another rail disaster on the scale of the Hatfield crash was "inevitable" because hundreds of unqualified workers were repairing tracks.

Dangerous rails like those that buckled killing four people on the day of the crash were no longer classed as a serious risk, he claimed, and lives were being put at risk every day because of the destruction of the safety culture since privatisation.

Loach's controversial new film, The Navigators, which will be shown on Channel 4 next Sunday, tells the story of how experienced railmen were replaced with "lumpen labour" from building sites who had no idea what they were doing. It culminates with one man being killed while working for a cowboy contractor.

The director claimed there was a thriving black market in personal track safety cards, the qualification workers need before they are allowed to work on the tracks. "They change hands in pubs for a few pounds. A highly skilled and vital job is just turning into another branch of the building industry.

"If people thought that their baker was driving their train they'd be very worried. Well, a lot of the people fixing the rails, arguably an even more important job, have no idea no what they are doing and no experience or training.

"When railmen tell you that they do not use their free travel passes anymore because they would rather not take the risk, you have to worry."

The Navigators, written by a railman Rob Dawber, who died earlier this year from cancer he contracted from working with asbestos on the railways, is the first film Loach has made for TV in 20 years. Loach, the maker of such award-winning films as Land and Freedom and My Name Is Joe, said rail workers had been cowed from going public about the dangers by "gagging clauses" in their contracts.

Railtrack, which contracts out rail repairs and maintenance to private companies, disputed Loach's claims and said "great progress has been made over the last year, al though there is still much to do". A spokeswoman insisted that Loach's allegations were up to six years out of date.

"The majority of track maintenance workers, more than 90%, have their relevant cards. We now have photo and hologramed cards and are about to introduce smart cards. The type of abuse he claims is going on is difficult to get away with, given our constant monitoring of contractors. Training has also improved."