Hacking suspect Neil Wallis may have provided "informal advice" to David Cameron's communications chief Andy Coulson before the general election, the Conservative Party has admitted.
Sources in the party said the advice had "nothing to do with the phone-hacking inquiry".
A spokesman insisted Mr Wallis was never employed by the Conservative Party.
The spokesman said: "There have been some questions about whether the Conservative Party employed Neil Wallis.
"We have double-checked our records and are able to confirm that neither Neil Wallis nor his company has ever been contracted by the Conservative Party, nor has the Conservative Party made payments to either of them.
"It has been drawn to our attention that he may have provided Andy Coulson with some informal advice on a voluntary basis before the election. We are currently finding out the exact nature of any advice.
"We can confirm that apart from Andy Coulson, neither David Cameron nor any senior member of the campaign team were aware of this until this week."
Shadow culture secretary Ivan Lewis said: "This revelation raises further serious concerns about David Cameron's judgment in appointing Andy Coulson.
"He must now come clean about Neil Wallis's role and activities in supporting Andy Coulson, both in his capacity as director of communications for the Tory Party, and then the Prime Minister."
Metropolitan Police commissioner Sir Paul Stephenson and his assistant commissioner John Yates, who both appeared before the Home Affairs Select Committee on Tuesday, resigned after being linked to Mr Wallis, deputy to Mr Coulson during his time in charge at the News of the World.