Portsmouth insists they did not breach Premier League rules over Portugal midfielder Manuel Fernandes.
League officials are currently investigating claims that Fernandes' six-month stay at Fratton Park contravened their regulations.
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'.
Benfica star Fernandes, 21, is part-owned by the Global Sports Agency. West Ham's signing of Argentinians 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.
But Portsmouth chief executive Peter Storrie said: 'We had a straight loan (arrangement) with Benfica. There was no third party involved.
'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.'
Storrie added: 'As I understand it, Everton are paying loan fees, something which we didn't do.'
Midfielder Fernandes quit Pompey at the end of his loan spell last month after they could not agree a fee with Benfica - thought to be £6million. Global Sports Agency are thought to have valued him at a similar figure, leading to speculation that Pompey would have had to shell out a total of £12million.
Like all Premiership clubs, they 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.'