The Government is seeking a High Court injunction to prevent border staff taking strike action on the eve of the Olympics, the Home Office said tonight.
The Home Office said the Government believed there was a "procedural error" in the ballot of members of the Public and Commercial Services (PCS) union and was turning to the courts to prevent the strike taking place.
It also repeated its calls for the PCS leadership to call off the strike ahead of the Olympics.
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, Home Office minister Lord Henley told peers that officials were examining the legality of the planned walkout, saying the decision to call the strike on Thursday was "opportunist and wholly unjustified".
He said just 12 per cent of the union's membership voted in favour of strike action in the row over jobs and pay.
"We are currently checking on the legality of the strike and, if satisfied it is illegal, we will take the appropriate remedies in the courts," Lord Henley said as he answered questions in the upper house.
Speaking before the legal action was confirmed, a PCS union spokesman said: "There are some very serious issues at the heart of this dispute - not least plans to cut 8,500 Home Office jobs, a third of the workforce.
"We can all see the damaging effect these cuts are having on the department's ability to function, whether at the borders or in the passport service.
"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."