IEP chairman Sir Stephen Irwin said: “The misconduct demonstrated here was significant.”
Mr Roberts faces being suspended from the Commons for six weeks.
However, his constituents are unable to trigger a recall petition because this is only permitted after a report by a Commons committee, in particular the Standards Committee, rather than the IEP.
Chris Bryant, Labour MP and chairman of the Commons Standards Committee, told BBC Radio 4’s Today programme: “In most lines of work, this kind of behaviour would lead to being sacked summarily.
“It seems extraordinary that voters won’t have an opportunity to take a view on that until the next General Election in the Delyn constituency.
“And actually, if I were Mr Roberts, I would not want to be exploiting a loophole in this way simply to stay in Parliament.”
He continued: “Our committee does not hear allegations of bullying and sexual harrassment. That is only done by the Independent Expert Panel and unfortunately up until now the Government has not changed the law so as to include decisions from the IEP for triggering the act.
“That’s a glaring omission. It’s a preposterous situation and the Government should change it as fast as possible.”
Mr Roberts is understood to have had the Tory whip removed following the Independent Expert Panel’s report.
Commons Leader Jacob Rees-Mogg is committed to bringing forward a motion on the suspension, Downing Street said.
The Prime Minister’s official spokesman said: “The Government is now considering next steps and will update shortly.
“There might be more from the Leader of the House who is committed to bringing forward motions to implement sanctions recommended by the independent panel.”
Asked about the loophole which means constituents might not be able to force a by-election, the spokesman said: “We are considering next steps and will update shortly.”
Mr Roberts has apologised for what he called a “breach of trust” over the unwanted sexual advances to a member of staff but added: “I will continue to do my utmost to serve my constituency.”
The complaint was made in June 2020 that Mr Roberts made “repeated and unwanted sexual advances” to the person who made the complaint and also made “inappropriate comments of a sexual nature and was overly intrusive about his personal life”.
The IEP report said Mr Roberts acknowledged that aspects of his behaviour towards the man who made the complaint were inappropriate, and offered an apology.
But he rejected the categorisation of his conduct as “sexual”, preferring the term “romantic”, the report said.
The case was investigated by the Parliamentary Commission for Standards which found Mr Roberts had breached the sexual misconduct policy.
The MP appealed to the IEP, first against the ruling and then against the proposed sanction.
The recommendation of a six-week suspension takes into account the “abuse of power or authority” by Mr Roberts as an aggravating factor.
But the IEP acknowledged that was going through “several challenges and significant changes in his personal life when he breached the sexual misconduct policy”.
Mr Roberts split from his wife and announced he was gay in 2020.
The panel said the sanction “reflects our view that the sexual misconduct found proved in this case amounts to a serious breach of the behaviour code which has significantly impacted the wellbeing of the reporter and has the propensity to undermine the legitimacy and authority of the House of Commons”.