Union members at the Driver and Vehicle Standards agency will go on strike next week after a breakdown in talks over pay and terms.
The DVSA – a Department for Transport executive agency which sets road safety standards and regulates the MOT vehicle testing system – set out plans for a more flexible employment contract last year.
But both the Public and Commercial Services union and Prospect have voted for walkouts, arguing that the terms are being forced through at the expense of staff and could undermine safety.
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Prospect members will take action from 8am to 11am on November 20, with an indefinite work to rule to follow. The union's negotiator Helen Stevens said her members had opted for strike action as a "last resort".
"The agency, supported by the Department for Transport, seems determined to railroad detrimental changes to terms, conditions and pay on its staff," she said. "Our hard-working DVSA members are frustrated by management’s intransigence. We have tried everything to resolve this and we do not want our action to have an impact on the public, but we have no choice."
PCS has meanwhile promised two days of strike action after 91.% of members backed a walkout on a turnout of 64.6%. The PCS walkout, from November 19-20, will be also be followed by action short of a strike.
"The union's aim is of course to resolve the issues in question by sensible, meaningful negotiation," a statement issued by PCS said.
"However, it is heartening that members are prepared to fight to defend their conditions. We are now strengthened by the powerful mandate of today’s ballot result."
PCS is particularly concerned that new working hours will lead to tests being carried out in poor daylight, and says the agency is already short-staffed.
DVSA chief executive Paul Satoor said it was "disappointing" that unions had opted for strike action and sought to reassure the public.
"Staff signed up to a new standard employment contract in April 2014 in exchange for a lump-sum payment and a 3 year pay deal," he said.
"This was agreed with the trade unions, and included transitional payments which came to an end on 1 November 2015. We have also recently offered operational staff a number of flexible working options to enable us to provide even more convenient and flexible services to our customers.
"It is disappointing that the trade unions have now chosen to oppose some aspects of the contract and the more flexible working options. We are doing everything we can to minimise any disruption to customers."
The DVSA came about through a 2013 merger of the Driving Standards Agency (DSA) and the Vehicle & Operator Services Agency (VOSA). According to the latest figures, the new agency employs just over 4,200 full-time staff.