The Public and Commercial Services union has announced that thousands of Home Office staff will strike on the eve of next week's Olympic opening ceremony.
The 24-hour PCS strike will coincide with one of the busiest periods in recent history for border control, as thousands of sporting spectators are due to arrive in the UK.
Home Secretary Theresa May has expressed her disappointment at the action.
She said: "I think that is shameful, frankly. They are holding a strike on what is one of the key days for people coming in for the Olympic Games.
"We will of course put contingency arrangements in place to ensure we can deal with people coming through the border as smoothly as possible."
Meanwhile, immigration minister Damian Green has reassured the public that contingency plans are in place.
