The Daily Mail reported claims that the names and addresses of people taking part in the research had been discovered and that postal staff “sifted through” mail to ensure it it was delivered promptly and correctly to those homes.

Postcomm said it is treating the matter as “confidential at this stage” but confirmed it was conducting a “preliminary assessment into a serious allegation against Royal Mail”.

Royal Mail said it had commenced an “immediate independent investigation” into the allegations and would be conducting interviews with staff in Glasgow to ascertain whether the claims were accurate.

The research project was commissioned from Research International. In a statement, the agency said: “The allegations, received via Postcomm, relate to the possibility of interference in the measurement of Royal Mail’s quality of service performance.

“The integrity of the quality of service measurement and the resultant data is something that Research International takes very seriously and we are working with Postcomm and Royal Mail to investigate this matter.”