An MP has been readmitted to the Conservative party, despite having been found to have sexually harassed a member of staff.

Rob Roberts had his party membership suspended for 12 weeks and was also banned from the House of Commons for six weeks, after an independent panel found he made repeated and unwanted sexual advances towards someone who worked in his office and used “his position as his employer to place him under pressure to accede”.

On Monday, Roberts regained his party membership, but still has the whip withdrawn, meaning he sits as an independent in parliament. However, the MP for Delyn in north Wales still sits on the government’s side of the Commons green benches, and votes in line with the Conservative whip.

A Conservative party spokesperson confirmed Robert’s readmission and that the remained withdrawn.

Although Roberts served his suspension from parliament, he avoided being subject to a recall petition. This is usually automatically triggered when an MP is suspended for more than 10 days. However, because Roberts appealed against the initial verdict by the standards commissioner, it emerged that his punishment was not covered by the existing recall legislation because the case was then judged by a separate body known as an “independent expert panel”.

The failure of Conservative headquarters “to stop Roberts’ readmission means that the withdrawal of his whip is in name only”, a senior Tory staffer told the Guardian. They added: “This sends a worrying signal to parliamentary staff members who are still dealing with predatory and abusive behaviour on a daily basis.”

The Labour party chair, Anneliese Dodds, said his readmission to the Tories was scandalous, and criticised the government for voting down a motion to change the law so the punishment of a recall petition could be applied retrospectively.

“Rob Roberts should have resigned as an MP the moment he was suspended from parliament for sexual misconduct,” Dodds said.” That he had now returned to the Conservative party showed it had let him off the hook, she added.

“Just last week, the government voted against allowing Roberts’ constituents to choose whether he is the right person to represent them, and now they are choosing to readmit him as a member of their party. Yet again, the Conservative party is acting like there’s one rule for their MPs and another rule for everyone else.”

The leader of the Commons, Jacob Rees-Mogg, has called on Roberts to do the “honourable” thing and step down as an MP, but argued that retrospective punishment would go against the principle that there should be no imposition of a sanction “which was not available at the time when a given case was determined”.

The investigators’ report into Roberts said that he had accepted that “aspects of his behaviour” towards a staff member were “inappropriate” and had offered to apologise but that he “rejected the categorisation of his conduct as ‘sexual’, preferring the term ‘romantic'”.