The union, which was due to meet Royal Mail today (28 June) in the High Court, said it had accepted its lawyers’ advice that it would be illegal to boycott access mail on the back of the consultative ballot alone.

A legal agreement had been reached, however "the fight goes on", the CWU said.

The ballot of 112,000 union members last month was held over alleged unfair competition from the likes of TNT and working conditions. The vote, on a 74% turnout, resulted in a 92% "yes" vote to the question ‘do you support the boycott of competitors’ mail?’

A CWU spokeswoman said: "Royal Mail has suggested there will be never be action but that’s not our understanding of the wording of the legal agreement. We will not take action on the consultative ballot but will explore other options. We may go for an industrial action ballot."

She said the finer legal points on acting on a consultative ballot were highly complex, made more so by the unique nature of the dispute. "Boycotting third-party mail has not been done before, but if we do want to take that route we have a great deal of support – 92%.

"At this stage we are not saying anything definite," she added. "An industrial action ballot could be for a strike or action short of a strike – there is a particular legal process to go through. But this is much further down the line; we’re not ruling anything in or anything out."

CWU deputy general secretary Dave Ward said: "We have accepted the advice of lawyers that it would not be legal to take action on boycotting competitors’ mail on the basis of the consultative ballot result alone. We are now considering how this action could be taken.

"In the meantime Royal Mail must explain to its workforce how they are going to protect revenue, jobs and terms and conditions when they have abjectly failed to tackle this issue. The government and regulator are sleepwalking into a disaster waiting to happen."

Royal Mail said it had received "legally binding undertakings" through the High Court from the union not to induce its members to refuse to handle access mail. Letters and parcels delivered under downstream access contracts accounted for around half of the daily post bag, it said.

"CWU held a consultative ballot to seek its members’ view on a number of issues including a threatened boycott of access mail. Furthermore we believe any action against access mail delivery would be unlawful.

"We are satisfied that Ofcom has put in place a framework sufficient to protect Royal Mail and the universal service from the impact of direct delivery competition."

Managing director for consumer and network access Stephen Agar said: "We need to deliver every item of mail that arrives in our network as the law requires. As a business or as an employee, we cannot be selective and treat one piece of mail any differently from any other."