Driving tests will be cancelled across the West Midlands with examiners to strike twice this week.
Examiners will walk out tomorrow and on Friday in a long-running dispute with the Driver and Vehicle Standards Agency (DVSA) over changes to working hours.
The action is expected to disrupt severely driving tests booked for both days, with staff at every testing centre in the region expected to take part.
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The latest strike action follows a national two-day walkout in November in a row over new working patterns which the Public and Commercial Services Union (PCS) said would lead to longer shifts.
Andrew Lloyd, Midlands regional secretary at the PCS, said the strike was forced when the DVSA threatened to dock the pay of staff working to rule.
He said: "The employer has provoked this strike. I can only describe their measures as deliberately provocative. The employer has forced this response from the union because of its response to working to rule.
"Ultimately, we are looking for a solution and we are bending over backwards but they seem determined to have a confrontation with the union."
Tomorrow's strike action will take place across the Midlands and on Friday all test centres across the country will be involved, according to the PCS.
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Tests booked to take place on those days will have to be re-booked although theory tests are not affected.
The union has called on the agency to conduct thorough research before making any changes, including into the "physical and psychological" effects of more tests and their likely impact on safety.
DVSA should also fully review staffing, given it admitted it was 350 posts short, the union added.
PCS general secretary Mark Serwotka said pushing examiners even further would be dangerous.
"The issues of road safety at the heart of this dispute are very serious and examiners deserve more than this petty-minded response from the DVSA," he said.
"It is shocking that the agency appears happy to press ahead without negotiating or fully understanding the likely consequences of its actions."
DVSA chief executive Paul Satoor said staff signed up to a new standard employment contract in April 2014 in exchange for a lump-sum payment and a three-year pay deal.
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"This was agreed with the trade unions and applies to all staff in the Department for Transport and motoring agencies," he added.
"The pay deal included transitional arrangements that came to an end on November 1, 2015. It is disappointing that the trade unions have now chosen to oppose the contract they agreed in 2014.
"We are doing everything we can to minimise any disruption to customers and we apologise for any inconvenience."