Mr Chris Patten, the chairman of the Independent Commission on Policing for Northern Ireland, has declined an invitation to testify to a US Congressional committee on the work of the commission.
Members of the commission will embark this month on a 10-day fact-finding mission to the United States.
Mr Patten has written to Mr Ben Gilman, chairman of the Committee on International Affairs of the House of Representatives, saying it would be "inappropriate" for him or other members of the commission to testify at the hearings of the committee about policing in Northern Ireland, which will probably begin in April.
Mr Patten said in his letter: "We are deeply aware of the sensitivity and delicacy of our work, and its importance to the success of the peace process as a whole . . .
"I am sure you will understand that, in the circumstances, it would be inappropriate for me or my fellow commissioners to testify to any outside parties. . .on the substance of our work, and none of us will be able to do so until our report is completed."
Commenting on Mr Patten's decision, sources on the International Affairs Committee said: "We hope that the commission's findings and recommendations will also reflect the independence Chris Patten is asserting in response to Chairman Gilman."
The commission has come under attack from California State Senator Mr Tom Hayden. He had asked to meet the members but was told to submit his concerns in writing, as there would not be enough time in the commission's schedule for such a meeting.
Mr Hayden said he was "extremely disappointed" that the commission was too busy to hear his testimony.
A spokesman for the police commission yesterday expressed surprise at Mr Hayden's criticism. He said he hoped a "mutually satisfactory arrangement" could still be arrived at.