Ansa Logistics, which transports vehicles across the country, said more than 2,000 cars had initially been locked at the depots in Avonmouth and Speke because their keys could not be found.

The company said it planned to seek an injunction in the High Court unless it received assurance that illegal secondary picketing will end at Avonmouth, Bathgate in Scotland and Portbury.

Ansa issued a fresh appeal to hundreds of strikers to understand the "reality" of a highly-competitive business, pointing out that its revenues were down 20% on the first quarter of this year.

"The payroll is the largest item in the company's cost base, and its fixed costs are twice those of the industry average.

"But this is not primarily about wages, it is much more about overmanning, productivity, and highly restrictive practices reminiscent of 1970s British industry," Ansa said in a statement.

About 350 drivers, fitters and support staff are involved in the action, which the union said was in protest at wage cuts and compulsory redundancies.