Royal Mail is seeking a High Court injunction to stop a postal strike, claiming that the ballot of workers had “potential irregularities”.
The company said it would make a formal application on Friday that the strike ballot “was unlawful and, therefore, null and void”.
A strike threatens to disrupt postal voting in the run-up to the general election as well as Christmas post.
The Communications Workers Union said it “refutes” Royal Mail’s claim.
The ballot of 100,000 Royal Mail staff was held over job security and terms.
Members of the Communications Workers Union (CWU) last month voted by 97% in favour of a nationwide strike, saying the company had failed to adhere to an employment deal agreed last year. Royal Mail rejects the claim.
In the company’s statement on Friday, Royal Mail said it had evidence of CWU members coming under pressure to vote “yes” in the ballot.
This included, the company claimed, union members “being encouraged to open their ballot papers on site, mark them as ‘yes’, with their colleagues present and filming or photographing them doing so, before posting their ballots together at their workplace postboxes”.
In a twitter post the CWU said “we clearly refute this”, adding it would make a fuller statement later on Friday.
A High Court hearing into Royal Mail’s application for an injunction is expected to be heard early next week.