Staff at car giant Ford in Merseyside are staging a 24-hour strike in a row over pay and pensions.
Unite, which represents about 1,200 white collar employees at the carmaker, said the walkout, from 6am, will affect sites across the country, including Halewood, Dagenham in east London, Bridgend in South Wales, and Southampton.
The union said staff were "furious" at plans to close the final salary pension scheme to new starters and lower their rates of pay from next year.
Unite national officer Roger Maddison said: "Our staff members will not stand by and allow Ford to create a two-tier workforce on pay and pensions. To date Ford has failed to make any genuine attempts to resolve this dispute.
"We fiercely oppose the closure of Ford's final salary scheme to new entrants because we believe ultimately Ford will try to close the entire scheme. Ford must prove that it is committed to the UK by investing in its UK workforce. The UK has the best sales in Europe, there's no excuse to attack the terms and conditions of a new generation of Ford staff.
"The company is also refusing to back away from creating a two tier workforce by making new starters work for less money for doing the same job as existing staff. This is totally unacceptable."
The dispute does not involve production workers, but Unite said it believed the walkout would affect production.
Picket lines will be mounted at Ford plants by the striking staff, who include foremen, IT workers and those based in administration.
A Ford spokesman said: "The issue giving rise to the industrial action relates to a disagreement between the company and a particular group of its employees in relation to their ongoing pay and benefit negotiations.
"Ford remains willing and available to continue discussions with the union representing these workers."