Portsmouth insist they did not breach Premier League regulations over their loan of Benfica star Manuel Fernandes.
Fernandes joined Everton on a six-month deal in January
Fernandes, 21, is now with Everton but began the season at Fratton Park.
League officials are investigating claims his loan stay at Pompey broke the rules because he is part-owned by a third party, the Global Sports Agency.
But Portsmouth chief executive Peter Storrie said: "We had a straight loan arrangement with Benfica. There was no third party involved."
According to Rule U18, England's top clubs are not allowed to field a player owned by a third party who would have the "ability materially to influence its policies or the performance of its teams".
West Ham's signing of Argentine duo Carlos Tevez and Javier Mascherano is also under scrutiny because of their link with Media Sports Investments, the company which tried to take over the Hammers.
As things stand, nobody is being accused of breaking the rule
Premier League spokesman Dan Johnson
Storrie is adamant Portsmouth are in the clear, and explained: "The Premier League have had all our documentation and, in fact, they helped to draw up the loan agreement because of our concerns over Manuel's injuries.
"The problem arises when a third party owns a player, but this was a straight loan. We have done nothing wrong and are not concerned by this.
"As I understand it, Everton are paying loan fees, something which we didn't do."
Fernandes left Portsmouth in January as they could not agree a fee for him with Benfica - and he completed his loan move to Goodison Park on transfer deadline day.
Like all Premiership clubs, Pompey have now been sent a written explanation of the rule from the Premier League and have a week to make comments before any further action is considered.
But Premier League spokesman Dan Johnson said: "We have had a number of enquiries about that particular rule and so we have sent a circular to all 20 clubs clarifying the issue.
"As things stand, nobody is being accused of breaking the rule."
Meanwhile, Serbian winger Ognjen Koroman has cut short his loan spell at Portsmouth to return to his homeland with Red Star Belgrade.
Koroman, 28, has made just three appearances this season - two as a substitute - for Pompey after arriving for his second loan spell from Russian club Terek Grozny in the summer.