The Post Office is planning to cut 150 jobs, many of them senior posts, according to Unite.
The union said the move came weeks after the company announced the appointment of more than 30 "highly paid" directors.
Unite officer Brian Scott said: "This appears to be a strategy to reward those at the top at the expense of the workers delivering the service.
"This is down to the lack of commitment from the government and a rickety management that does not have a coherent strategy going forward.
"As the shareholder, the government should be investing in new products and services in the Post Office that allows it to grow, prosper and become sustainable for the benefit of the public."
A Post Office spokesman said: "We are creating a leaner and flatter structure and this means we are making changes at all levels of the business.
"If changes are proposed, our people would always be the first to know.
"The term director reflects the increased spans of control which have been introduced for a number of people.
"These changes are not about promotions or pay increases."