The workers, who are members of the Public and Commercial Services (PCS) union, plan to walk out for two days at the end of July. They have threatened further action next month if necessary.

The industrial action would cause significant delays for people applying for or renewing passports, the union claimed.

The employees have previously rejected an imposed pay freeze for staff earning £21,000 a year and above. For those earning less, Steria is understood to have offered a pay rise below the level of inflation that is equal to an average of around £180 pro-rata over a nine-month period.

More than 160 workers on the passport offices contract are paid just £13,500 a year, according to PCS.

"These private sector staff provide an invaluable public service for UK citizens applying for or renewing their passports.