The Rugby Football League have confirmed they are studying an “incident” in Leeds’s 13-12 defeat by St Helens at Headingley last Friday.
The governing body declined to give details or identify the player involved, but Hardaker has been accused on social media of blowing a kiss at opponent Mark Flanagan, before allegedly making an offensive remark.
The Yorkshire Post understands no complaint was made on the night, but the investigation was launched after the game was viewed by the RFL’s match review panel.
Hardaker has played in Leeds’s last seven fixtures since returning from a five-match suspension imposed over a homophobic comment made during a defeat at Warrington Wolves in May.
A spokesman for the governing body confirmed: “The RFL are investigating an incident from last Friday’s game.”
Hardaker has not been charged, but if the matter is taken further he could face a ban of up to eight games, which would end his season with Leeds and cast doubt over his availability for England’s Four Nations tour Down Under.
The player defended himself on Twitter yesterday through a series of messages in which he dismissed the allegations as “a load of rubbish”, though those remarks were later deleted.
Contacted by The Yorkshire Post, Hardaker declined to comment.
Rhinos’ management stood by Hardaker during his ban earlier in the season and pledged to work with the player to ensure there was no repeat.
Both Hardaker and coach Brian McDermott travelled across the Pennines to take part in training sessions with gay rugby league club Manchester Canalsiders.
Hardaker, 22, joined Rhinos from Featherstone Rovers in 2011 and was a member of their Grand Final winning teams later that season and the following year.
He also played a starring role in last month’s Tetley’s Challenge Cup win over Castleford Tigers.
He has been recalled to England’s elite training squad, after an early departure from last year’s World Cup campaign.
The investigation will be a concern to Leeds, whose second-choice full-back, Ben Jones-Bishop, will be sidelined for the rest of the year due to an anterior cruciate ligament injury.
Rhinos will be without Australian centre Joel Moon for the final two games of the regular season after he was last night suspended for a Grade C shoulder charge in the match five days ago.
Moon, who was sent off following the clash with Saints’ Mark Percival, opted to appear before a disciplinary hearing, rather than submitting an early guilty plea.
The former Salford player admitted the offence and was fined £300 on top of the suspension, which rules him out of Friday’s trip to Wigan Warriors and the following week’s fixture at Hull.
Wigan’s Liam Farrell was found not guilty of making a Grade B reckless high tackle on Joe Mellor of Widnes Vikings in the 50th minute of the game.
The decision means Farrell will be available for selection against Leeds on Friday.