The Government Investment Corporation is set to become a “significant” shareholder in the postal service, Britain’s Sky News said.
It did not say how many shares it had ordered ahead of the initial public offering, which is estimated at up to 3.3 billion pounds (Bt165.7 billion).
Britain’s ruling Conservative-Liberal Democrat coalition is selling up to 70 per cent of the state-owned Royal Mail, which dates back to 1516.
Trading is expected to start in the middle of October.
Postal workers have threatened to strike in protest.