The arrest of Tory MP Damian Green as part of a Home Office leak inquiry was "not proportionate", a report said.
Former British Transport Police chief Ian Johnston said the leaked material only amounted to "embarrassment matters" for the Government.
He said the documents did not contain information which threatened to undermine national security or Government effectiveness.
A separate review by Her Majesty's Chief Inspector of Constabulary, Denis O'Connor, found that police should only investigate the most serious leaks.
Mr Johnston was brought in by Scotland Yard Commissioner Sir Paul Stephenson last December to review an inquiry into Home Office leaks. His secret interim report was published for the first time as part of an overall review ordered by former home secretary Jacqui Smith.
In a heavily censored document, the former chief constable accepted that the arrests of Mr Green and his civil servant mole Christopher Galley were lawful. But he said there was a "strong question mark" both over the police decision to arrest the Tory immigration spokesman and the manner in which it was carried out.
Mr Johnston said: "In my view, the manner of Green's arrest was not proportionate because his arrest could have been carried out on an appointment basis, by prior agreement, and when he could be accompanied by his legal representative. I recognise the significant political context in which the leaks occurred and the professional anxiety they caused within the Civil Service.
"However, I regard the leaks for which Galley can be clearly held responsible in law, as amounting to 'embarrassment matters' for Government. I do not think, from the material presented to me, that the leaks in themselves are likely to undermine Government's effectiveness."
The arrest of Mr Green and police searches of his Commons office and home in November last year provoked outrage among MPs. He was held for nine hours by officers from the Met's specialist operations branch and questioned at Paddington Green police station.
The Crown Prosecution Service (CPS) announced in April that neither man would face criminal charges. Director of Public Prosecutions Keir Starmer said the damage to the Home Office was not excessive and the material was of legitimate public interest.