Billy Hayes, Communication Workers Union: "Today is a referendum"
The result of a ballot of 120,000 Royal Mail workers that could pave the way for a second national strike in two years will be announced later.
The Communication Workers Union (CWU) says it is confident of support for industrial action in the dispute over modernisation of the service.
Royal Mail says it is losing money on letter and parcel delivery and has warned of the damage strikes could do.
Downing Street called on both sides to settle the dispute.
The Prime Minister's spokesman said nobody would benefit from industrial action, adding that modernisation was "still urgently needed".
Royal Mail's operations director, Paul Tolhurst, told the BBC that the company wanted to keep talking to the CWU.
"The union is constantly demanding that we halt the changes we are making. But we have completed our changes for this year, so there is a clear opportunity for the union to stop striking," he said.
By Martin Shankleman, BBC employment correspondent
Here we go again. That's likely to be the reaction of many people on hearing the news that the UK is heading towards the possibility of another national post strike, the second in two years.
Even trying to fathom the precise cause of this dispute is difficult.
Both sides cite a vast range of grievances and issues, ranging from Royal Mail accusing union members of blocking the introduction of new technology, and refusing to work their contracted eight hours; to unions complaining about members being forced to work excessive hours and a culture of management bullying.
Ironically, the only common ground between the two sides seems to be that they both agree that Royal Mail is in a financially parlous state, and a strike would only make a grim situation even worse.
But the general secretary of the CWU, Billy Hayes, said that workers had little choice but to vote for action in the pay and job cuts dispute.
"It has come to this because of the management of Royal Mail's and the government's refusal to engage the workforce on modernising the company," he said.
"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 to see strike action, we want to see a negotiated settlement," he added.
Mr Hayes also criticised the government for "walking away" from the Royal Mail's pension fund deficit.
The result of the vote is expected to be announced at 1230 BST.
Royal Mail made an operating profit of £321m in the year to 31 March, but it was the first time in 20 years that all four parts of the business had gone into the black.
These changes are absolutely essential if Royal Mail is to survive
The company says the number of letters and parcels its core business delivers is falling by 10% each year, losing it £170m per annum.
Both the Royal Mail and the CWU agree that job cuts are needed as part of a vital streamlining of the mail service to cope with the drop off in postal deliveries.
But the CWU says Royal Mail managers are refusing to meet its demand for a signed agreement determining the scope of cuts, and job and pay security guarantees for those workers who will ultimately remain in their jobs.
The Royal Mail says that by signing up to an agreement after the last national postal strike in 2007, the CWU accepted the future job cuts it needed to make in order to modernise the service.
'No going back'
Mark Higson, Royal Mail's managing director, said there could be "no going back" on efficiency measures.
David Small explains the impact of the strike on blind people
"These changes are absolutely essential if Royal Mail is to survive in an increasingly tough communications marketplace," he said.
Dave Ward, deputy general secretary of the CWU, argued that modernisation should be about improving services rather than cutting them.
"Postal workers are striking to defend future services as well as for jobs and modern conditions," he said.
"Modernisation is crucial to the future success of Royal Mail, but the implementation of change must be agreed and it must bring with it modern pay and conditions."
The union would have to give seven days' notice of any strike action.
BBC employment correspondent Martin Shankleman said: "Royal Mail is urging the union to pull back, warning strikes can only damage customers and future prospects for the business."
Our correspondent added that estimates of the mail backlog created by the series of regional strikes in recent months varied from five million items to 25 million, "depending on whether you believe the company or the union".
Meanwhile, online retailer Amazon has said it is looking at its options as it tries to avoid any disruption to deliveries caused by potential Royal Mail strikes in the run-up to Christmas.
"With the possibility of strike action in the near future, we have been working on contingency measures with our other carriers," an Amazon spokesman said.
However, he denied that the company had cancelled any long-term contracts with Royal Mail.
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