UK government officials have confirmed they will seek a High Court injunction in order to prevent border staff from going on strike on the eve of the London 2012 Olympic Games.
Citing a “procedural error”, the government said a ballot of members of the Public & Commercial Services (PCS) was illegal.
The government also repeated its call for the strike to be called off.
A Home Office spokesman told the BBC: “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.”
In response, the union said it would robustly defend its right to strike.
Commenting, a PCS 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.
“It is our intention to robustly defend any legal challenge.”