Sir Ian strongly insists he was open about his friendship with Mr Miller
Auditors are examining Scotland Yard contracts awarded to a company owned by a friend of Sir Ian Blair, head of the Metropolitan Police.
The Met Police Authority (MPA) is researching a contract given in 2002 to the consultancy firm Impact Plus, owned by Sir Ian's friend Andrew Miller.
Sir Ian insisted he was "open and straightforward" about his relationship with Mr Miller.
The MPA and the Met police said Sir Ian was not under investigation.
A spokesman for the MPA said: "MPA Internal Audit is aware of concerns about single tenders concerning Impact Plus and is currently researching the matter."
In November 2002, Sir Ian sent a letter to the Treasurer of the MPA which detailed his friendship with Mr Miller.
Sir Ian said: "I was open and straightforward in informing both the MPS and MPA about my friendship with Andy Miller prior to any contract being awarded.
"I strongly reject any suggestion that I have behaved inappropriately in any way and consider I have acted with absolute probity in these matters."