John Bercow is facing calls to resign as Speaker of the House of Commons after fresh allegations of his “bullying” behaviour came to light, as Theresa May said she was “concerned” by the reports.

The Daily Telegraph has learned that Mr Bercow reportedly apologised to two members of Parliamentary staff, including a postman, after they complained following angry outbursts on the phone.

It came as sources told this newspaper that Mr Bercow's poor behaviour towards staff had been an "open secret".

One source who did not want to be named said: "He had a reputation for being unkind and bullying people who worked in his office, even very senior staff did not enjoy working with him.

"On one occasion he could be heard shouting and swearing in his office and a female clerk emerged looking stunned."

Another added that the Speaker was "quite feared" and that staff worried about retribution if they spoke out against him.