The merger of AS Eesti Post and its only competitor, AS Express Post, was barred by Estonia's Competition Authority for a second time on Wednesday.
Both companies deliver mail and periodicals, but Express Post only operates in cities. A merger would give AS Eesti Post a monopoly.
The agency blocked a similar bid in 2011.
Külliki Lugenberg, head of the Competition Authority's merger control department, said the act was not justified despite fewer people using postal services.
"In the Competition Authority's opinion, it is not justified to give permission for a merger that would immediately eliminate all competition in the periodical home delivery service, business letter service and direct mail forwarding service," she said.
Editor: Helen Wright