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: "The lives of staff have been made intolerable by these cuts and they're at breaking point.
"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."
In advance of the announcement David Cameron said he hoped the strike would not go ahead but insisted the Olympics would be safe and secure regardless.
Speaking at a press conference in Kabul, he said: "I do not believe it will be right, I do not believe it will be justified."
Tory MP Dominic Raab, told the Daily Mail that the strike was "self-indulgent, reckless and deeply unpatriotic".
‘There is no credible basis for this vindictive strike action – a militant minority led by a union head-banger should not be allowed to paralyse the Games.’
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."
Earlier, Labour leader Ed Miliband spoke out against disrupting the Olympics, saying: "People should not be striking during the Olympics. People should not be disrupting the Olympic Games."