The Communication Workers Union (CWU) said this morning it is confident of support for local strikes to become nationwide.
The general secretary of the CWU, Billy Hayes, told the BBC: "We are very confident. All the signs are that [postal workers] will support the union on what is a referendum on the management of Royal Mail."
"We don't want a strike, we want a negotiated settlement," he added.
The news comes as Royal Mail was reported by The Times to have lost a £25 million contract with Amazon, its second largest customer. However Amazon issued a statement last night denying it had cancelled the contract but admitting it was making contingency plans with alternative carriers for the run-up to Christmas.
"We have not cancelled any long-term contracts with the Royal Mail. They continue to be one of a number of carriers we use," Amazon.co.uk said in its statement.
Royal Mail has been beset by a series of wildcat strikes since July in a dispute over the modernisation of the service, pay, conditions and pensions.
The CWU says it accepts the need for change, such as the introduction of new sorting technology to help counter the threat of competition, but claims that Royal Mail is not consulting the union about new working practices.
Direct marketing clients are increasingly worried about the reliability of Royal Mail and the havoc the strikes are causing to activities related to direct mail such as contact centres.