Hundreds of Border Force officers at Heathrow Airport have started a four-day strike.
Members of the Public and Commercial Services union (PCS) walked out at 05:00 BST in a dispute over working conditions. It will end on Thursday.
The PCS said more than 300 members would go on strike, affecting Heathrow terminals 2, 3, 4 and 5.
The Home Office says it is "disappointed" in the strike action but is open to "discussing a resolution".
Heathrow Airport previously said it was supporting the Home Office with "contingency plans".
A spokesperson said: "eGates will be open as usual, and most journeys should be unaffected. However, some longer queue times may be experienced."
'No option'
The PCS said the action, which is due to end at 07:00 BST on Thursday, was over plans to introduce new rotas that would see about 250 of members "forced out of their jobs at passport control".
General secretary Fran Heathcote said: "It's disappointing that, despite talks last week, the Home Office is not prepared to grant any flexibility to their new roster.
"None of our dedicated and highly experienced members in the Border Force want to take strike action but the way they've been treated by their employer leaves them with no option."
A Home Office spokesperson said: "The changes we are implementing will bring the working arrangements for Border Force Heathrow staff in line with the way staff work at other ports, provide them with more certainty on working patterns, and improve the service to the travelling public.
"We have robust plans in place to minimise disruption where possible, but we urge passengers to check the latest advice from operators before they travel."
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