Unions say thousands of driving tests have been cancelled as more than 1,000 examiners joined a strike by office staff at the Driving Standards Agency.
About 5,000 driving tests could be cancelled
The strike has added to what is already the worst outbreak of industrial unrest in the civil service for 13 years.
The 48-hour Public and Commercial Services Union strike follows the collapse of pay talks.
The Department for Work and Pensions said 43,000 staff walked out on Monday. The union said it was up to 80,000.
On Tuesday evening a DSA spokesman said: "Initial indications show that around 1,000 of the 1,500 driving examiners in the UK took part in the industrial action.
"This resulted in around 5,000 driving test cancellations, out of around 7,000 booked tests."
DSA chief executive Gary Austin had earlier urged people with driving tests scheduled on Tuesday to attend as planned.
"Not all the examiners are members of the Public and Commercial Services union," he said.
"And even if they are, we cannot be sure they will not turn up for work," he said.
The DSA said that if candidates found there was no examiner to test them, a new date would be automatically booked for them.
But one woman from York whose test was cancelled was angry at how much she would now have to pay to book another date.
"I'm really annoyed about the fact that it's been cancelled," she said.
"The situation in York with the tests is that I will have to wait another 14 weeks and pay for another three months of lessons. That's going to cost me another £300."
The driving examiners' strike followed union claims of pay inequalities of £4,000 a year between DSA staff and those doing the same job in the Department for Transport.
The 48-hour strike in the DWP, the biggest government department, is a result of "appalling" levels of pay and an unacceptable appraisal system, the union said.
The DWP said nearly 90% of job centres remained open on Monday, despite the "completely indefensible" strike.
But the PCS said between 80% and 90% of its members had walked out of job centres and benefits offices, disrupting services.
It added that support for the strike remained "strong" on Tuesday.
Strikers' anger intensified after a leak on Monday of a report from the head of a government efficiency review suggesting up to 80,000 civil service jobs could be cut.
Later, shadow chancellor Oliver Letwin caused further controversy by suggesting 100,000 jobs could go.
PCS general secretary Mark Serwotka said the union would fight any move to cut jobs as "totally unacceptable".
The civil service had become an "easy target" for politicians, he added.
"Civil servants are sick of the lack of recognition for their hard work," Mr Serwotka said.
He added that a quarter of civil servants earned less than £13,750, with starting salaries in the DWP as low as £9,900.
Call for talks
The PCS said pay negotiations in several other government departments remained unresolved and it could not rule out further ballots for industrial action.
A total of 130,000 civil servants had already taken industrial action over pay, were about to strike or could be involved in strikes, the PCS said.
Mr Serwotka called on the Department for Work and Pensions (DWP) to restart pay talks.
"We have written to the DWP's permanent secretary, Richard Mottram, today (Tuesday) saying we stand ready to negotiate with him," he said.