Passengers arriving at UK ports and airports may face delays today and tomorrow, as border control staff have gone on strike.
Some 2,500 staff who work for Border Force, part of the UK Border Agency (UKBA), have staged a walkout in protest over changes to rosters and roles.
PCS general secretary Mark Serwotka said: "Far from saving money, these plans will increase costs unless there are also job cuts, and we believe this is what UKBA is planning.
"Our members perform vital roles in keeping our borders secure and we can not accept cuts or changes that put this at risk."
According to union PCS, staff are unhappy about the "downgrading of highly sensitive immigration control work," as it would "detrimentally affect the agency's ability to detect forgeries and could endanger national security if less skilled and experienced staff undertook this vital work".
The union said changes to shift patterns of border staff will increase costs and could put national security at risk.
The UKBA wants to change flexible working practices currently in place, said PCS, which means staff are allocated to passport control on a daily basis, and can provide round the clock coverage where necessary.
This is of particular benefit in emergencies, said the union, most recently during the volcanic ash crisis.
According to the UKBA, staff taking part in the strike represent a minority, who work at the primary control point of UK border controls.
Lin Homer, UKBA's CEO, said: "This strike action is fundamentally about the agency's efforts to modernise its working practices to deliver a more efficient and effective service."
www.ukba.homeoffice.gov.uk www.pcs.org.uk