The GPO, which ran the nation’s phone system, instructed its staff to “stonewall” Ms Pankhurst by ignoring her letters, the correspondence and cuttings in the BT Archives reveal.
The campaigner, who was earlier revealed to have been the subject of MI5 surveillance, wrote to the Postmaster General with concerns that duplicate lines opened the door to “improper use by unscrupulous persons”.
Her fears had been fuelled by a news story about a gynaecologist who had been struck off following an affair with a patient.
The relationship was exposed by the patient’s husband, who arranged with the GPO to duplicate the phone line at his house in order to intercept his wife’s calls.
Dr Sarah Jackson, at Nottingham Trent University, who discovered the letters, said: “Buried in the files were two letters from Pankhurst about an extension line that had been installed in her home.
“The first response from the Post Office indicated that this was a normal extension line, but Pankhurst knew it wasn’t because she already had a normal extension.
“The timing of the 1930s, when MI5 were actively monitoring her behaviour, suggests she knew she was being monitored.”
Dr Jackson said: “It is quite clear that the Post Office knew about the extension line. These concerns are not new. People talk about spying being the second-oldest profession.”