Passport Office staff will strike today over staff shortages.
Passport workers say the shortage of staff is to blame for the serious backlog in processing applications this year.
Extra staff were drafted in to deal with the backlog of 30,000 applications, and Home Secretary Theresa May last month issued an apology to those affected by the delays. Words:PA
The Passport Office said it had faced the highest demand for applications in the last 12 years, adding that predictions for the summer surge were "substantially exceeded".
HM Passport Office chief executive Paul Pugh said earlier this month that the 775,000 applications received in June "I think is the highest ever recorded in a single month".
Today's strike by members of the Public and Commercial Services (PSC) union is an attempt to draw attention to staff shortages dating as far back as 2010 as well as inequalities in pay, the union claimed.
It said the high-profile nature of the recent applications backlog has forced the office to take the issue more seriously, and cited recent figures from immigration minister James Brokenshire which showed staff numbers were down by more than 300 in the past four years.
PCS general secretary Mark Serwotka said: "The staffing crisis in the Passport Office has been obvious for everyone to see and it shouldn't have taken a committee of MPs to force the chief executive to
meet us to discuss it.
"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 Home Office criticised the union for the timing of the strike, saying it could affect people's summer holidays.
"We are disappointed the PCS leadership has called this strike given that HMPO has been in discussions with the union on their issues of concern," said a spokeswoman.
"Holding a strike at this stage is irresponsible and will only inconvenience our customers 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.
"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."
The spokeswoman said customer service counters will remain open today.
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