Thousands of Passport Office workers are to strike in a row over staff numbers, which union officials claimed have caused backlogs in processing applications this year and pay.

The Public and Commercial Services Union, which represents members walking out on 28 July, said the agency only agreed to meet the union to "seriously discuss jobs" after recent media and political scrutiny, and under pressure from the House of Commons home affairs select committee.

A new recruitment drive is underway but the PCS said many of the posts being advertised are not permanent jobs and are only likely to replace the 8% of staff – around 300 full time equivalent posts – that the agency forecasts will leave every year anyway.

"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," said Mark Serwotka, the general secretary of the PCS.

"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."

But the agency said it had faced the highest demand for applications in the past 12 years.

The union is also in dispute over pay in the Passport Office, where the PCS claim that staff can be paid up to £3,000 ($5,092, €3,792) less than their colleagues doing similar work in other areas of the Home Office.

A Home Office spokesperson said: "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. 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.

"If necessary, we have contingency plans in place to ensure that during any industrial action passport offices will maintain a service to the public and keep any inconvenience to a minimum. All HM Passport Office Customer Service Centre counters will remain open and we will be able to issue passports for those in emergency situations, with urgent travel needs."