Thousands of staff at the Home Office, including airport immigration workers, are to stage a 24-hour strike the day before the opening of the Olympics in a row over jobs, pay and other issues.
The Public and Commercial Services union (PCS) said its members will walk out on July 26, and will take other forms of industrial action, such as a ban on overtime, from July 27 to August 20.
The action will hit border controls at ports and airports including Heathrow, threatening disruption to people travelling to London for the Games.
The union warned it will announce further action if ministers continue to "refuse" to negotiate an agreement, warning that job and spending cuts are hitting services to the public.
The strike will involve staff across the Home Office, including the UK Border Agency, the Identity and Passport Service and Criminal Records Bureau.
PCS general secretary Mark Serwotka said: "Ministers have known about these issues for a very long time and need to act now to sort out the chaos they have caused. They're acting recklessly in cutting so many jobs and privatising services, and are provocatively refusing to talk to us with a genuine desire to reach an agreement."
Home Secretary Theresa May branded the PCS strike decision "shameful". "I think that is shameful, frankly," she said in a round of broadcast interviews. 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."
Immigration Minister Damian Green said the low turnout in the strike ballot, which saw just one in five vote, showed most union members did not back a walkout.
He told BBC Two's Newsnight: "PCS members don't want this strike to happen; seven out of eight of them didn't vote for a strike. They are patriotic people, they care about their job, they care about the reputation of this country. It's a small group in the union leadership that is behaving disgracefully, trying to make political capital out of the Olympics, which is meant to be a great national celebration."
He said previous action failed to cause significant disruption at Heathrow, with enough staff available to replace strikers.