Olympic visitors to London could face long airport queues as border staff, and other Home Office personnel, vote to go on strike in a dispute over pay and cuts.
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The Public and Commercial Services (PCS) union, which represents Home Office staff among others, voted to strike for 24 hours next Thursday - the day before the official opening of the Olympic Games on 27 July.
Hundreds of thousands of visitors are expected in London every day while the Games are under way.
East Midlands train drivers also plan to take industrial action over the Olympics, after a vote by Aslef union.
The PCS said members would take other forms of industrial action on key dates over the games. Staff involved work for the UK Border Agency, the Identity and Passport Service and the Criminal Records Bureau.
The PCS is in an ongoing dispute with the Home Office over pay and job cuts and said on Wednesday that its members backed its motion to hold a strike. But the government said the 20 per cent turnout at the ballot box invalidated the union's decision, and that it is unlikely to have public support.
Of those who voted, 57 per cent backed a strike, and 75 per cent backed other forms of industrial action.
Staff shortages at airport passport desks in and around London have already been blamed for long queues at immigration, with some passengers having to wait for several hours.
Government 'papered over the cracks'
"Ministers have known about these issues for a very long time but have chosen not to act. "The lives of staff have been made intolerable by these cuts and they're at breaking point," PCS general secretary Mark Serwotka said in a statement.
We believe they have acted recklessly and irresponsibly in cutting so many jobs and, in the case of UKBA (UK Border Agency), they have simply tried to paper over the cracks by deploying severely undertrained staff at our borders. Mark Serwotka, PCS general secretary
"Ministers have known about these issues for a very long time and need to act now to sort out the chaos they have caused.
"We believe they have acted recklessly and irresponsibly in cutting so many jobs and, in the case of UKBA (UK Border Agency), they have simply tried to paper over the cracks by deploying severely undertrained staff at our borders."
The strike by East Midlands train drivers, over a dispute about pensions, will take place on 6 to 8 August.
The government has already been forced to draft in extra staff at passport control to cope with the Olympic rush. And following G4S's failure to meet its contractual obligations to deploy enough security staff, the government has called on at least 3,500 military personnel to work on Olympics security.
Strike branded 'shameful'
Immigration Minister Damian Green called the decision to strike "completely unacceptable".
"The security of the UK border is of the utmost importance and we will use our trained pool of contingency staff to ensure we minimise any disruption caused by planned union action," he said. "Any action that disrupts the Olympics will be completely unacceptable and the public will not support it."
Home Secretary Theresa May branded the PCS decision to strike on one of the key Olympic dates "shameful".
John Cridland, CBI director general, said: "As the world arrives in London for the Olympic Games, every one of us should be giving our guests the warmest possible welcome. For PCS to go on strike on this key day beggars belief."
Labour leader Ed Miliband said: "People should not be striking during the Olympics. People should not be disrupting the Olympic Games."