Ed Miliband has urged border staff not to strike on the eve of the Olympics.
The Labour leader said he was "totally against" next Thursday's planned walkout, the day before the Games officially open.
Thousands of staff at the Home Office, including airport immigration workers, are to stage a 24-hour strike in a row over jobs, pay and other issues. The Public and Commercial Services union (PCS) said its members 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.
But Mr Miliband, who has been criticised for his close links to the unions, branded the strikes "totally wrong".
He said: "I don't think they should be happening. Nothing must be allowed to disrupt the Olympic Games. I implore anybody involved in these strikes not to go ahead with them. I don't think it is right for the country, I think it will cause disruption and I don't think it's what people want to see."
Speaking on a visit to Corby in the East Midlands, he added: "It certainly won't win any sympathy from the public for the cause that people who are thinking about going on strike are putting forward."
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."