The Conservatives confirmed on Monday morning that Roberts was a Tory party member again after a 12-week suspension.
The MP for Delyn in North Wales will continue to sit as an independent as the Tories are still withholding the party whip in the House of Commons.
A Conservative Party spokeswoman said: “Rob Roberts’ membership suspension concluded on Monday November 1 after serving a 12-week suspension. The whip will remain suspended.”
Deputy Labour leader Angela Rayner said the move was a “disgrace”, adding: “Rob Roberts has no place in Parliament, politics or public life.”
Labour Party chair Anneliese Dodds described the scheduled end of Roberts’ suspension as “scandalous”.
“Rob Roberts should have resigned as an MP the moment he was suspended. That he is now set to return to the Conservative Party shows they’ve let him off the hook,” she added.
Roberts was stripped of the whip after Parliament’s Independent Expert Panel found he broke the sexual misconduct policy by making repeated and unwanted advances to a man.
The former staff member who made the complaint in June last year told BBC Wales that the MP had repeatedly propositioned him, leaving him feeling “uncomfortable”, “shocked” and “horrified”.
Roberts, who became an MP in 2019, apologised for the “completely improper” behaviour but insisted his actions were “romantic” rather than sexual.
The Commons approved a motion to suspend Roberts from the House for six weeks in May, in line with the recommendation from the independent panel.
The Conservatives suspended him from the party for twice that duration, while senior figures including Commons Leader Jacob Rees-Mogg urged Roberts to do the “honourable” thing and stand down as an MP.