The decision escalates the confrontation between the union and Royal Mail over pay and the future of thousands of jobs.
The union has refused to accept Royal Mail's imposition of a 2.9% pay rise on staff and its plans to modernise the company, including issuing shares to workers. It fears that 40,000 staff could lose their jobs under the plans.
The union said it is keen to negotiate a settlement with Royal Mail and will not officially give notice of its decision to hold a ballot until July 3, leaving a week in which it hopes to hold talks with the management.
If it gives notice on July 3, it would then despatch ballot papers to its 136,000 members on July 10 with the ballot to close on July 31.
A Royal Mail spokesman said the company was very keen to involve its staff and the unions in the modernisation process.
"That is why we have invited all the front-line reps and managers to meet this week with the chairman and chief executive to go through these challenges and talk about how we can face up to them together," he said.
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