Thousands of passport workers will join a strike in a row over staff shortages as efforts to clear an ongoing backlog continued.
The Passport Office said the highest demand for applications in 12 years caused the long delays in processing passports.
Extra staff had to be drafted in to cope, while Home Secretary Theresa May was forced to apologise to those affected.
The Public and Commercial Services Union (PCS) said poor staffing levels were partly responsible for the backlog of passport applications, claiming numbers had dropped by more than 300 since 2010.
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