Royal Mail’s procedures state employees cannot open their mail at delivery offices without the prior authorisation of their manager.
It comes after Conservative MPs warned that any major disruption to postal voting may increase Labour’s chances of electoral success and leave the final election result open to legal challenge.
Studies have shown that elderly people are more likely to request a postal vote, with nearly half of Tory voters being aged 65 or over.
In an interview with the Daily Telegraph, Andrea Leadsom, the Business Minister, said that the “timing of this threat is clearly politically calculated” and it would be a “taste of things to come” if Jeremy Corbyn were to be elected as Prime Minister.
CWU lawyers dismissed these claims, arguing that there was no evidence of interference with the ballot.
Mr Justice Swift, who is chairing the case, said he hopes to give his ruling on Wednesday.
If the Royal Mail wins, the union may not have time to strike before the election scheduled for December 12.