Hundreds of passport workers across the UK have gone on strike in a dispute over staff numbers and pay.
The Public and Commercial Services Union (PCS) said the action was a "bid to end staffing shortages that have caused the ongoing backlog crisis".
Home Office data suggests about 360,000 applications are being processed - but it is not clear how many are overdue.
The Home Office said 875 PCS members had walked out and warned the action could jeopardise people's holidays.
The 24-hour strike is taking place at all eight of the UK's Passport Office sites - Belfast, Durham, Glasgow, Liverpool, Southport, London, Newport, and Peterborough - and will continue until midnight.
BBC News correspondent Richard Lister said the Passport Office had "struggled to cope with an unprecedented number of applications" this year.
"With the summer holiday season now well under way the agency is still under enormous pressure to keep up with demand," he said.
In June the Home Office redeployed hundreds of staff to deal with a growing backlog of applications, amid reports of people waiting up to two months for passports that are meant to be processed within three weeks.
The government has said the number of full-time equivalent staff at the Passport Office fell from 3,700 at the end of 2010 to 3,164 two years later, but then rose to 3,333 by the end of 2013.
The Home Office estimates that around 170,000 passports are processed per week.
PCS said the office had "cut hundreds of staff since 2010" and only agreed to "seriously discuss jobs after recent media and political scrutiny".
It said there had been talks in recent weeks but senior Passport Office staff were "still failing to commit to work with the union to agree a long-term solution to understaffing, instead of the sticking plasters they are currently applying".
PCS general secretary Mark Serwotka said: "We are still a long way off getting a commitment from the agency that it will work with us to put the proper resources in place to ensure these backlogs do not reoccur year after year."
The union is also in dispute with the government over pay, claiming Passport Office workers can be paid up to £3,000 less than people doing "similar work" in other parts of the Home Office.
"The agency appears to have accepted the argument over pay but would still have to put a business case to the Treasury for funds," PCS said.
The "threat of more privatisation" is also part of the dispute, the union added.
The spokeswoman said service counters will remain open today.
Meanwhile, the Home Office said holding a strike at this stage was "irresponsible" and would inconvenience passport applicants and "jeopardise their holidays".
"We strongly urge PCS to reconsider this action which is not in the interests of staff or the general public they are dedicated to serving," a spokeswoman said.
"HM Passport Office staff know how important it is to hard-working people and their families to receive their passports in time for their summer holidays. They have shown this through their hard work and commitment during this exceptional period of high demand."
Appearing before a committee of MPs earlier this month, Passport Office chief executive Paul Pugh said 170,000 passports were being issued each week, and he expected that to rise to about 180,000 a week "over the summer".
Speaking on 8 July, he said the number of passports classed as "work in progress" had totalled 508,000 the previous week, but that the number was falling.
On Sunday the Home Office said the latest figure was 360,000, but it did not say how many had taken longer than the standard three weeks.
It said the backlog was falling, but the PCS strike could put holidays at risk for people who were waiting for passports.
It added that customer service counters would remain open at passport offices.