The Rugby Football League are investigating a claim that Hull fielded an ineligible player in their Carnegie Challenge Cup fifth-round win over Widnes.
Hull re-signed prop forward Jamie Thackray from Leeds on March 27, more than three weeks after the initial cup deadline, and he played in their 32-18 victory over the Vikings on Sunday.
Thackray, who was a member of Hull's Cup-winning team in 2005, was also a tryscorer in his team's 42-5 fourth-round success at Rochdale on April 20.
If the East Yorkshire club, who have been drawn away to Bradford in the quarter-finals, are found to have breached the League's bye-laws, they could be fined or even expelled from the competition.
"We are aware of the issue and are looking into it," confirmed RFL spokesman Craig Spence.
A Hull spokesman said the club had no comment at this stage but Widnes admitted they would be delighted to take Hull's place in the next round.
"We are aware that Hull played an unregistered player and we're waiting to see what the League have to say," said Widnes operations manager Pat Cluskey.
"We will abide by whatever decision the League make, as we did with our nine-point deduction."
Widnes point out that they left two players, Richard Fletcher and Sam Barlow, out of their team because they were signed from Whitehaven and Huddersfield respectively after the cup register closed.
"We did a lot of work prior to the game checking the eligibility of our players," added Cluskey. "Had those two players been available, it may have been a different result."
The RFL introduced a second deadline to register players for the Challenge Cup following the decision to move the competition to later in the season three years ago.
Players must be registered by Monday of the week leading up to the third round to be eligible to play in the next three rounds while players signed up to the Monday before the quarter-final stage are free to play in the rest of the competition.
Hull could escape with a fine if found guilty of breaking the rules but there are precedents in other sports for a tougher punishment.
In 2004, Wasps were thrown out of rugby union's Powergen Cup for fielding an ineligible player, hooker Jonny Barrett, who had already played for Henley in the competition, and beaten sixth-round opponents Bristol took their place in the quarter-finals.
In December 2006, Bury were expelled from football's FA Cup for fielding an ineligible player, Stephen Turnbull, in a 3-1 win at Chester, who were then reinstated.
And earlier this year, Brechin were thrown out of the Scottish Cup for fielding two ineligible players in a fourth-round replay against Hamilton.
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