Home Secretary Theresa May has rejected an accusation of a conflict of interest in Lincolnshire Police's decision to award a £200m contract to G4S.
At the Police Federation conference, Mrs May was asked about Tom Winsor, a partner of a law firm which advised the security company on the deal.
In 2010, Mr Winsor was appointed by the government to author an independent report on police reform.
Mrs May told the conference there was no overlap between Mr Winsor's roles.
In February, Lincolnshire Police agreed a deal to pay G4S £200m over 10 years to deliver a range of services, including human resources, finance and IT.
Mr Winsor, whose review proposed changes in how the police are paid, is a partner at White and Case but the firm said he played no part in its work advising G4S.
Before joining the firm Mr Winsor spent five years as a rail regulator.
The home secretary was asked about his appointment by delegate Sarah Adams at the conference in Bournemouth.
Ms Adams said: "When you appointed Tom Winsor to carry out your independent review of policy, did you know that the law firm Tom Winsor is part of, which is White and Case, was negotiating the multi-million groundbreaking deal for G4S with Lincolnshire Police?
"How can it be fair and independent if there's a vested interest?"
Mrs May said: "Tom Winsor did his review entirely independently. He did not do that review as part of the firm - he did it as an individual.
"You might not like all the answers that came out of the Winsor Review but there is a process whereby the federation's voice will be heard in response to these proposals."
Barry Young, chairman of Lincolnshire Police Authority, agreed there had been no conflict of interest.
He said: "My understanding is the work he's done for the government in relation to his report on pay and conditions was as Tom Winsor and not the firm White and Case. I see no conflict of interest whatsoever."
A spokesperson for White and Case said: "The firm rejects any suggestion of a conflict of interest between Tom Winsor's independent police pay review and any of the firm's clients.
"The police pay review was undertaken by Mr Winsor in his personal capacity and who was appointed, in such capacity as an impartial reviewer, by the home secretary."
A spokesperson for G4S said: "There has been absolutely no conflict of interest: Mr Winsor has not been involved in any capacity with the legal team which advised us on our contract with Lincolnshire Police.
"Furthermore no member of the G4S policing team has even had contact with Mr Winsor."