Allegations of a security failure at MP Jared O’Mara’s office are being investigated by the House of Commons.
The BBC has learned the Sheffield Hallam MP’s chief of staff, Gareth Arnold, was not vetted by Parliament.
Mr Arnold, who is running the MP’s office in his absence, had access to sensitive casework and the MP’s email account on the parliamentary network.
The House of Commons said it had taken steps to “prevent unauthorised access to Mr O’Mara’s parliamentary account”.
During an investigation by BBC podcast, The Next Episode, Mr Arnold, 26, said: “I am running a constituency office on behalf of an MP without the required security clearance from the parliamentary authorities. It’s crazy isn’t it?”
Mr O’Mara has said he is “taking time out” to deal with his “mental health and personal issues” after it emerged he sent sexually inappropriate messages to a female employee.
The BBC also found constituents were being blocked on Twitter by Mr Arnold, who sent antagonistic responses and memes to their queries.
Others say casework was not being dealt with, which Mr Arnold denies.
He said: “People have joked to me that I was basically the MP for Sheffield Hallam… that horrified me”.
Since the BBC published the podcast, Mr O’Mara’s Twitter account and website have been taken down and when the BBC visited his office on Monday morning, it found the premises had been left vacant.
Sinead Parkinson, a founding member of the Hallam Constituents Facebook group, said: “It’s positive that Parliament are investigating the breach of security but it does not change the fact that Hallam constituents are still without an MP.
“His staff have evacuated the office, he has taken down his Twitter, and there is no phone or email communication. Action needs to be taken as we have no representation.
“Unless Jared O’Mara resigns we are left in this hopeless situation.”
Tom Brake MP, who sits on the House of Commons commission in charge of the administration of the House, told the BBC: “These are disturbing revelations. Whatever pressures MPs’ offices are subject to, constituents are entitled to be treated with respect.”
In July, the Mr O’Mara announced plans to resign from his seat days after Mr Arnold publicly quit his role by posting his resignation statement on Mr O’Mara’s Twitter account.
Reporting team: Dino Sofos, Sam Bonham and Poppy Damon, The Next Episode