BBC News
By James Williams
image captionNick Ramsay was elected as an AM for Monmouth in 2007
The Tory MS for Monmouth is understood to be the fourth Senedd member named in an inquiry into the drinking of alcohol on Welsh Parliament premises during the Wales-wide alcohol ban.
Three have already apologised over the incident in December.
But several sources have told BBC Wales Nick Ramsay is the fourth MS referenced by the probe as having drunk alcohol in a licensed tearoom.
Mr Ramsay said he has resigned from the party's front bench.
The move comes ahead of an investigation by the standards commissioner.