Senedd member Alun Davies has re-joined Labour's group in the Welsh Parliament following his suspension for drinking on parliament premises during an alcohol ban.
He apologised at the time of his five week suspension but denied breaking the Covid-19 rules in early December.
The Blaenau Gwent Senedd member will continue as Labour's candidate for the seat in May's Senedd election.
Tory MS Darren Millar also stepped down as chief whip, whilst the party's Senedd member (MS) for Monmouth Nick Ramsay is understood to be the fourth Senedd member named in an investigation.
The Senedd Commission investigation found that five individuals - four MSs and the Conservative group's chief of staff at the time - consumed alcohol in the Senedd's licensed tearoom on 8 December.
Four days earlier a ban on the serving of alcohol in pubs and licensed premises across Wales had been imposed, leading the investigation to conclude the incident could amount to "a possible breach" of Covid-19 regulations.
Senedd's Presiding Officer Elin Jones said the matter had been referred to Cardiff Council and the parliament's own standards watchdog.
A spokesman on behalf of the Standards Commissioner said: "The law prohibits the commissioner from admitting or denying that a complaint has been received and from making any comments in relation to any complaint that has been submitted."
Cardiff Council said their investigation was still underway.
In a statement released at the time of his suspension, Alun Davies said: "I am very sorry if my actions have given the impression that I am in any way not committed to upholding the regulations which I have consistently supported throughout the last year."
He said the purpose of the meeting "was to seek to persuade the Welsh Conservatives to support my proposal for a Welsh Hearts Bill, which the Senedd endorsed on 21 October, and to make a commitment to enact this life-saving legislation in their manifesto for May's election".