More than 1,000 Passport Office workers will go on strike for five weeks over a dispute about jobs, pay and conditions, unions say.
Members of the Public and Commercial Services union working across England, Scotland and Wales will take part in the action from 3 April to 5 May.
The union warned of delays to applications and the delivery of passports in the run-up to summer.
Those working in Belfast are being balloted and could join the strike.
BBC News has contacted the Home Office for a response.
More than 4,000 people are employed by the Passport Office across the UK, meaning that according to the union figures around one in four workers will be walking out.
The offices affected in England, Scotland and Wales will include Durham, Glasgow, Liverpool, London, Newport, Peterborough and Southport.
At peak times – including the month of April when the strike will take place – the Passport Office can receive 250,000 applications per week, according to travel expert Simon Calder.
It means that more than one million applications could be sent during the strike period, he said.
The government says the strike does not affect their guidance which is still to allow up to ten weeks to get a passport.
A UK adult passport is valid for 10 years, but if it is due to expire you may be refused entry into some countries.
Most countries require the passport you are travelling with to be valid for the duration of your planned stay, but other countries – mainly ones where you need visas for entry – require a passport with a certain period of time left until the expiry date. Guidance for each country can be found on the government’s foreign travel advice website.
PCS general secretary Mark Serwotka said the dispute was over a proposed 2% pay rise which he said was not being increased any further by the government.
He added the strike action had come about because “ministers have failed to hold any meaningful talks with us, despite two massive strikes and sustained, targeted action lasting six months”.
He said: “Their approach is further evidence they’re treating their own workforce worse than anyone else.
“They’ve had six months to resolve this dispute but for six months have refused to improve their 2% imposed pay rise, and failed to address our members’ other issues of concern.”
He went on to say that the government was “ignoring our members” over their pay concerns but this would not make them “go away”.
Last year, hundreds of thousands of people were affected by passport processing delays as coronavirus travel restrictions were lifted.
The latest action from passport office workers comes after months of strikes over pay disputes in other sectors, such as rail workers, London Underground drivers, teachers, NHS staff, regional BBC journalists, university lecturers and civil servants.
On Thursday, unions representing healthcare workers in England agreed a final pay offer with the government – which if accepted by members at a vote would bring walkouts from nurses, ambulance crews, paramedics, hospital porters and other health workers to an end.
Passport delay warning as five-week strike called