Postal workers say the company has failed to stick to a pensions deal agreed last year.

Royal Mail Plc said on Friday it would apply for an interim order from the High Court to block a potential strike by its biggest union, the Communication Workers Union, saying it believed the strike ballot had been unlawful.

The United Kingdom‘s postal service company said it is worried by the potential for industrial action around the general election on December 12 and in the traditionally busy run-in to Christmas.

The CWU last month had voted by 97 percent in favour of a nationwide strike, saying the company had failed to adhere to a pension deal agreed last year. Royal Mail said it had honoured the deal.

“Royal Mail have made an application to take us to the High Court. They claim there are irregularities with our ballot. We clearly refute this and will be represented. A hearing will possibly be on Tuesday,” the CWU said in a tweet on Friday.

The CWU, which has not said when it would hold the strike, had on Monday rejected an offer that Royal Mail said was aimed at avoiding action that could affect mail deliveries ahead of the general election.

Royal Mail has told the union that if it removed the threat of strike action for the rest of 2019, the company would enter talks without preconditions. CWU, in turn, called Royal Mail’s offer a “stunt” which the union would not fall for.

Royal Mail expects its application to be heard in the High Court in the week starting November 11.