Labor MP Conor McGinn has had the whip suspended after a formal complaint was made against him.
It is understood Mr McGinn, who represents St Helens North, has also been administratively suspended from the membership of the Labor Party pending an investigation.
No details of the complaint or what it relates to have been confirmed.
A spokesperson for Labor said they could not comment on individual cases.
Mr McGinn has been an MP since 2015 and previously served as the party’s deputy national campaign co-ordinator, seeking to help Labor win more seats at elections.
He has also sat on the front bench as a shadow Cabinet Office minister and a shadow Home Office minister, as well as holding roles in the shadow whips office.
The MP becomes the fifth from Labor to face investigation under the party’s new complaints process, with all of them sitting as independent MPs while probes are carried out.
Sky News has approached Mr McGinn for comment.