British Airways is seeking an injunction to block a proposed strike by cabin crew, claiming that the ballot supporting industrial action was invalid.
The airline is also trying to ascertain how many of its cabin crew members might want to work normally during the 12-day period affected.
BA had requested that the Unite union abandon its strike plan by 14:00 today but the carrier says the union has not complied. It states that it will seek an injunction to halt the strike, which is set to begin on 22 December.
Unite could not immediately be reached to comment on the BA decision.
BA says it has contacted the union three times since 11 December, pointing out possible problems with the ballot. It believes that "irregularities" - the nature of which it has not detailed - render the ballot void.
"We are absolutely determined to do whatever we can to protect our customers from this appalling, unjustified decision from Unite," says BA chief Willie Walsh.
BA says its managers are working to establish the number of cabin crew members who wish to continue working over the strike dates. The carrier describes this action as a "back-up" to its legal proceedings.
It reiterates that it is prepared to hold talks, without pre-conditions, with the union in order to resolve the dispute over new staffing measures - already the subject of a separate court action, due to be heard in February 2010.