The turnout in the ballot of 115,000 CWU members was less than two-thirds (63%), of which 78% voted in favour of a strike versus 22% against.
A 24-hour walkout will be held on Monday 4 November if no agreement has been reached by that date, after the CWU allowed two weeks for an agreement to be reached with Royal Mail's management.
The CWU is in dispute with Royal Mail Group over "pay, pensions and the impact of privatisation on job security [and] terms and conditions" and has pledged that the dispute will remain until "each and every demand made has been met".
All bar 371 Royal Mail employees that opted out of their free share options are currently sitting on paper profits in excess of £3,500, although they will not be able to sell their shares for three years from the date of the IPO.
Dave Ward, CWU deputy general secretary, said: "Postal workers have spoken very clearly that they care about their jobs, terms and conditions far more than they care about shares. The stakes have become much higher for postal workers since privatisation making this ballot more important than ever. Postal workers will not be the people who pay for the profits of private operators and faceless shareholders.
"We have said from the beginning that we want an agreement and we still do. The question now is whether this privatised Royal Mail still wants an agreement. We have offered the company a two week period to reach an agreement and having already had many hours of negotiation, this is achievable if there is a will. The clock is ticking for both sides and we need Royal Mail to work to reach agreement before this deadline.
"What we want is a groundbreaking, long term, legally binding agreement that not only protects postal workers’ job security, pay and pensions – but will also determine the strategy, principles and values of how the Royal Mail Group will operate as a private entity.
"This means there will be no further breakup of the company, no franchising of individual offices or delivery rounds, no introduction of a cheaper workforce on two-tier terms and conditions and no part-time industry.
"It will mean – regardless of who owns Royal Mail – this company will not be able to enter the race to the bottom and replicate the employment practices and service standards of their competitors."
Meanwhile, the CWU has also announced that it will hold a second ballot on the issue of "boycotting competitors mail" to supplement the current strike action over jobs, terms and conditions and pay.
The boycott of mail from downstream access (DSA) providers, such as TNT, UK Mail and Citipost, which is delivered to Royal Mail's sorting offices for final mile delivery was first mooted in December last year.
The dispute centres on the fact DSA providers pay lower wages than Royal Mail and can cherry-pick the most profitable routes; however, it ignores the fact that Royal Mail is free to set the price it charges for DSA providers to access its network.
Postal workers voted in favour of a boycott of DSA mail in a ballot in March, drawing criticism from Royal Mail, which pointed out that DSA mail accounted for almost half of the mail it handled and contributed annual profits of £80m.
The dispute over the boycott of DSA mail went to the High Court in June with the CWU eventually backing down and accepting its lawyers advice that it would be illegal to boycott access mail on the back of the consultative ballot alone.
However, the union believes that its members could legally boycott access mail on the back of a ballot for industrial action, resulting in today's ballot announcement; Royal Mail has warned in the past that it operates in a "very competitive market" and that any boycott could result in a "significant loss of business".