Border guards at British airports will walk off the job on July 26, the day before the London Olympics begin, creating an Olympic-sized logistical headache for British officials.
Members of the Public and Commercial Services Union voted for the 24-hour strike in a dispute over pay and job losses.
Even without the strike, London's Heathrow Airport has been beset for months by sporadic long lines at passport control, which the union blames on government spending cuts.
The problem had eased in the past week as thousands of Olympic VIPs arrived for the games, but a walkout threatens a return of the endless waits at the worst possible moment for Britain's international image.
Home Secretary Theresa Maycalled the decision to stage a strike on the eve of the games "shameful".
She said the government would "put contingency arrangements in place to ensure we can deal with people coming through the border as smoothly as possible".
During previous border guard strikes in November and May, the government drafted in managers and public servants - including Prime Minister David Cameron's press secretary - to help staff immigration desks and minimise the disruption to incoming travellers.
The government - eager to avoid any disruption as visitors descend for the July 27-August 12 games - is hoping many staff turn up for work despite the walkout. Only 20 per cent of union members voted in the strike ballot.
The union said its members also will take other forms of industrial action, such as a ban on overtime from July 27 to August 20 - what is expected to be one of the busiest periods ever for London's airports.
In more transport trouble, about 400 train workers have threatened to strike for three days in central England during the Olympics in a pension dispute.
The strikes could be highly disruptive to the games - Olympic soccer matches are being held across the UK, many workers and Olympic tourists are coming from outside London and thousands of spectators are likely to use trains to get to venues around the capital.