The two are due to return to the High Court on Friday 28 June in the latest turn to a dispute that followed a ballot of 112,000 union members last month over alleged unfair competiton 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?’. But at court on Wednesday 19 June the CWU undertook not to up the boycott ante for at least a week.

A union spokeswoman said: “Royal Mail had already given notice they intended take this court action before they knew the result of the ballot. It was very jumpy about the boycott element of the ballot.

“In effect they tried to take out an injunction on something we hadn’t done, but we had agreed with Royal Mail we would not do anything for seven days. There was no injunction, so this is a bit of a nothing.”

However a boycott was still possible, she insisted: “There’s little I can say until next Friday. There could be another hearing, but that’s crystal ball gazing. It depends on what happens between now and then.

“I really don’t know what will happen next. This has never been done before; nobody has ever suggested boycotting particular types of mail, it’s a complete unknown and we can’t say anything.

“But a boycott is still possible. The overwhelming majority of members at Royal Mail have spoken in favour of that action and they are ready and willing to do that.”

A spokesman for Royal Mail said he could not comment on an ongoing legal process, but a prepared statement from the organisation said: “CWU has in the High Court given an undertaking not to induce its members employed by Royal Mail to refuse to handle access letters and parcels. This is pending a further hearing at the High Court on Friday June 28.

“As a result any direction by the Communication Workers Union that postmen and women should refuse to handle Access letters and parcels would be unlawful.

“Royal Mail is fully committed to the delivery of all of the mail entrusted to us and we have consistently made clear we would take all appropriate steps to prevent any disruption of this.”

Under Downstream Access contracts, Royal Mail accepts letters and parcels at its mail centres that have been partially sorted by large customers and other postal operators and delivers to homes and firs for an agreed price. This accounts for around half of the Royal Mail daily post bag and is big business.

The spokesman said: “The CWU has cited concerns over competition in direct delivery as a rationale for any action. Royal Mail made a detailed submission to Ofcom describing our concerns over the potential threat to the Universal Service posed by unfettered direct delivery competition.”