Postal workers have voted strongly in favour of taking industrial action over pay, in what would be the first national postal strike since 1996.
Royal Mail recently asked for a stamp price rise to cut its losses
About 77% of Royal Mail's Communication Workers Union (CWU) members who voted in the ballot supported the move.
The union is in dispute with the Royal Mail over its 2.5% pay offer.
A series of walkouts will now be held by about 130,000 CWU members, unless new talks can lead to a breakthrough in the dispute.
The union said workers' pay should rise to the national average over the next five years. Royal Mail says this amounts to £1bn and is unaffordable.
The ballot result was announced at the union's annual conference in Bournemouth.
"Unacceptable" cuts in jobs and postal services as well as "attacks on postal workers' terms and conditions" were the key issues of the dispute, said CWU deputy general secretary Dave Ward.
"Postal workers have rejected the company's arguments, rejected their plans for the future and delivered overwhelming votes in support of their union.
"We say to the public the threat to the very future of your postal services is real."
The union is legally obliged to give Royal Mail seven days notice of any industrial action.
Mr Ward added that no date would be set for strike action at this stage.
"We will give the Royal Mail a further opportunity to back off from their cuts and come back to the negotiating table with a fresh approach."
But he added: "The size of the yes vote and Royal Mail's hostile behaviour makes this an even more serious dispute."
Royal Mail says it has made a "realistic and fair" offer which includes a 2.5% increase in basic pensionable pay.
UK POSTAL MARKET FACTS
Businesses send about 87% of all mail
Only 10% of mail is household to household, with 3% posted by the public to business
The 500 biggest firms account for 50% of all mail sent
There are now 18 licensed mail operators, including Royal Mail, TNT Post UK, DHL Global Mail and UK Mail
The offer also comprises an £800 share dividend and a 50% share in any savings above budget in an employee's local unit.
Postal services minister Jim Fitzpatrick warned workers that a national strike would harm their cause - and could damage the Royal Mail.
Staffs should instead look to solve their differences through "constructive discussions" instead, he said.
'Need to modernise'
The union held three ballots, covering 127,000 Royal Mail workers as well as two separate groups of employees involved in pay and jobs disputes.
About 5,000 of union members working in Post Offices were balloted in protest at closures, pay and moving post offices inside WH Smith stores.
And cash handlers, who deliver money to branches, also voted on a strike.
A Royal Mail spokesman said that the ballot result "will not change the absolute need for Royal Mail to modernise, which is in the interests of everyone in the company."
He added: "Mail volumes have already fallen and more people are using electronic communications."
The company's rivals were already 40% more efficient than Royal Mail, the spokesman added.
James Greenbury, chief executive of rival firm DX Group, said he expected to add more customers in June alone than in the last 12 months if postal workers do go ahead with the proposed strike.
Damage to small business
The Federation of Small Businesses (FSB) warned that a postal strike would severely damage the UK economy and called on both sides to negotiate a solution.
"Small businesses would be hardest hit by this dispute," said FSB trade and industry chairman, Clive Davenport.
"They employ over half of the private sector workforce in the UK and are totally reliant on the Royal Mail. Other mail providers do not offer an alternative to small firms because of their size.
"A cheque delayed in the post can mean the difference between life and death for a small business, which means that this strike cannot be allowed to go ahead."