Eric Joyce slated over alleged relationship with girl, 17

Ms Lamont claimed Mr Joyce had “abused his position” following the latest allegations against the Falkirk MP, who has already been suspended from Labour after being charged with assault following an incident in a bar at the Houses of Parliament.

Ms Lamont said she was “disgusted” about the reports involving Mr Joyce, who was previously criticised for being the highest-claiming MP at Westminster and for using taxpayers’ cash to buy oil paintings for his constituency office.

The former army major, who was dismissed from Labour’s front-bench after a drink-driving conviction, was reported to have formed a relationship with his former researcher Meg Lauder, when she was just 17 and still at school.

He was said to have met Ms Lauder, now 19, after she helped during his 2010 general election campaign that saw him returned to Westminster. She appeared on a campaign leaflet beside the MP and was described as a student. At the time, she was still a pupil at Graeme High in Falkirk.

The teenager also worked in Mr Joyce’s constituency office.

The MP, a father of two, separated from second wife, Rosemary, before the May 2010 election.

Ms Lamont yesterday attacked her party colleague as “unfit” to hold office, as it emerged she is set to announce a shake-up in the way Scottish Labour candidates are selected.

She said: “If these reports are true, this is a man who has abused his position of power and authority. I’m disgusted, regardless of any other issues. I think this makes Eric Joyce unfit to stand for the Labour Party.”

Mr Joyce has denied the allegations about his private life, adding he did not have a sexual relationship with the teenager.

He said: “The story is not true and I will be taking legal advice.”

He has faced calls from his constituency Labour Party in Falkirk to “do the decent thing” and resign as an MP.

Mr Joyce is accused of assaulting Stuart Andrew, the Tory MP for Pudsey, in the Stranger’s Bar at Westminster.

It was claimed that Mr Joyce had been drinking for much of the day and allegedly lashed out after Mr Andrew asked him to quieten his singing.

It was also alleged that he struck a Labour whip, Phil Wilson, and other MPs as they tried to calm him down, and that he had to be restrained by five police officers.

Scottish Labour last night confirmed Mr Joyce would remain suspended from the party, as well as the group of Labour MPs at Westminster.

A spokesman said: “Eric Joyce was immediately suspended from the Labour Party when serious criminal charges were brought against him last week, so processes have already started. That case is now sub-judice, but Eric Joyce remains suspended.

“These separate revelations are highly disturbing, and Johann has made her position very clear.”

Mr Joyce’s predecessor as the area’s MP, Dennis Canavan, who was expelled from the Labour Party for standing as an independent for the Scottish Parliament, said the politician was not “a fit and proper person” to be a member of parliament.