Postal workers could return to the picket lines in December and disrupt the vital Christmas post, a union has warned.

Five days of stoppages over a period of three weeks brought heavy disruption to Royal Mail services last month, as Communication Workers Union (CWU) members protested over pay, conditions, and controversial modernisation plans.

A further two days of strike action were avoided with an eleventh hour announcement on Thursday that the two sides had agreed an interim deal to end the dispute.

It is hoped the deal, final details of which will be thrashed out in a series of meetings over the coming weeks, will end the threat of potentially damaging disruption over the Festive period.

Postal workers, however, have threatened a return to industrial action if the deal fails.

Trevor Flowers, CWU spokesman at Watford’s Ascot Road sorting office, warned that the result of an earlier ballot of members (which made the strikes legal) still stands and could be re-enacted if necessary in December.

He said: “The mandate for strike action still holds. The original ballot by members is still live so we could return to industrial action after “Of course nobody in their right mind wants to take more industrial action. We don’t want to cause inconvenience to the public and businesses but we need a decent agreement out of this and we need to preserve the postal service for the public.”

Mark Higson, Managing Director of Royal Mail Letters, said: “We look forward to positive and constructive discussions on the next stage of Royal Mail’s modernisation plan, which is key to the future of the company, all those who rely on it, and to the future of the Universal Service."