The Home Office has said that it will go to court to seek an injunction preventing a strike by border staff which has been called for the eve of the Olympic Games.
The government believes that there has been a "procedural error" in the ballot of members of the Public and Commercial Services (PCS) union.
A Home Office spokesman said:
"We believe that there was a procedural error in the PCS ballot and are therefore seeking an injunction at the High Court to prevent PCS taking strike action on Thursday. We want the PCS leadership to call off this irresponsible strike and we continue to ask members not to walk out at a time when the eyes of the world are on the UK."
Earlier the Home Office Minister Lord Henley had told peers that the strike was "opportunist and wholly unjustified".
The PCS union has defended the strike, and a spokesman emphased that there were "very serious issues at the heart of this dispute - not least plans to cut 8,500 Home Office jobs, a third of the workforce."
The spokesman said that the effect of cuts on the ability of the border and passport services to function was already being damaged by cuts, and said the union would robustly defend any legal challenge.a
However he added: "Our preference is to resolve these by negotiation, and we would hope ministers would rather sit down and talk to us, instead of going to the courts."
When immigration staff went on strike over pensions last year, civil servants were drafted in to prevent the dispute affecting travellers.
Heathrow airport is anticipating its busiest week ever in the run up to the Olympic Games opening ceremony on Friday.
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Troops called up
The news of the government's intervention came as Culture Secretary Jeremy Hunt confirmed that some 1,200 troops who had been put on standby for the Olympics last week will now be deployed. Mr Hunt said that although the numbers of staff to be provided by the private security firm G4S continued "to rise significantly", ministers would leave nothing to chance, adding:
"It's better to have those troops on the ground so that were they to be needed they can swing into action immediately."
Locog Chief Executive Paul Deighton said: "With three days to go, we just want to make sure this works without any worries at all ... The reason that we've chosen to deploy the extra 1,200 is, sure, the plan is to go from 6,000 (G4S staff) to 7,000, but we can't be absolutely certain about anything with a temporary workforce."
Mr Deighton revealed that roving teams would go around to check and, if necessary, train people on the job: "it's not just a question of training them and sending them out there."
The extra troops are on top of an additional 3,500 drafted in a fortnight ago.
In addition, the Metropolitan Police Assistant Commissioner Chris Allison, who is also the national Olympic security co-ordinator, said that on peak days there would be some 12,500 police officers on Olympics duty across the country, 9,500 of them in London. These would be on top of the officers providing normal policing across the capital.
Mr Allison said that training had been cancelled and leave reduced to allow this.