The former Hartlepool MP Mike Hill has been found to have breached Parliament's sexual misconduct policy.
An independent panel said he would have faced a significant sanction had he not resigned from his Hartlepool seat in March.
The resulting by-election saw the Conservatives take the seat from Labour.
Mr Hill is also facing an employment tribunal case, with a decision expected to be made next month.
An Independent Expert Panel (IEP) found former Hartlepool MP Mr Hill committed two breaches of the rules in relation to his behaviour towards a woman who is a former parliamentary worker.
Mr Hill resigned as an MP before the panel could impose a sanction for his behaviour.
But the IEP report said that had he not resigned "we would have likely considered recommending the sanction of suspension from the House".
The IEP was considering an appeal following a report by the Parliamentary Commission for Standards which found Mr Hill had acted in breach of Parliament's Sexual Misconduct Policy in respect of one of the three allegations made by the woman.
Both Mr Hill and the woman appealed and the IEP found that he had actually breached the rules in two of the three allegations.
Lawyer Suzanne McKie QC said "there can be no proper excuse for Hill's behaviour"
Sir Stephen Irwin, chairman of the IEP has said: "The sub-panel took a very serious view of his conduct, and had he remained a Member of Parliament, a significant sanction would have been under consideration.
"In the light of his resignation, however, the sub-panel concluded that no available sanction met the facts of this case and the specific circumstances of the Responder. They therefore did not impose or recommend a sanction."
Mr Hill was notified of the panel's decision to reject his appeal on March 5 and he resigned on March 16.
Commons Speaker, Sir Lindsay Hoyle, has decided that Mr Hill should be denied a parliamentary pass as a former member.
Mr Hill's resignation led to the Hartlepool by-election, which saw Labour lose the seat to Tory Jill Mortimer.
An employment tribunal has also been held into the case, with a decision likely to be made towards the end of June.