Downing Street has denied allegations it had a hidden e-mail system from which messages were deleted after the cash-for-honours inquiry began.
Tony Blair has been questioned by police during their investigation
ITV News said police had been alerted to a second network in Downing Street.
But a Number 10 spokesman said that there was no second system containing details of party donors.
Police are investigating whether money was donated to political parties in exchange for peerages - all those involved deny any wrongdoing.
ITV News said the information had been revealed in recent weeks, possibly inadvertently by a witness, and had led to the arrest of Downing Street political adviser Ruth Turner last week.
But the prime minister's official spokesman said: "This story is untrue. There is no second computer system at No10.
"There are no e-mails of the type that seem to be described. The police have had full access to the system and full cooperation and the police have not put any of this to us."
The police inquiry began after it emerged that a number of large secret loans had been made to the Labour Party before the 2005 general election, and that some of those lenders had subsequently been nominated for peerages.
The investigation has since widened to cover the other main parties and officers have spoken to about 90 people, including Tony Blair and former Conservative leader Michael Howard.
Four people have been arrested - Ms Turner, Labour's chief fundraiser Lord Levy, Labour donor Sir Christopher Evans and head teacher Des Smith, who was involved in the government's City Academy programme.
No-one has been charged. Police were expected to submit a file on the case by the end of January to the Crown Prosecution Service, which will decide if any charges are brought.
But after Ms Turner's arrest they said more time was needed before the investigation was complete. Ms Turner has said she "absolutely refutes" any wrongdoing.