The decision by London police to arrest Conservative lawmaker Damian Green and raid his office in the House of Commons last year raised questions about the historic rights of Parliament in Britain.
Police and soldiers have been traditionally forbidden to enter Parliament without permission since King Charles I sent in troops to arrest a group of lawmakers in 1642.
Ian Johnston, a former head of British Transport Police, said in his independent report ordered by Metropolitan Police chief Paul Stephenson that police could have handled the case differently.
"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," he said.
In a separate report, the chief inspector of the constabulary, Denis O'Connor, said police should only investigate the leaks of highly sensitive government documents.
The government broke with protocol when it asked police to investigate possible leaks after documents were released revealing that illegal immigrants had been granted security guard licenses and that an illegal immigrant was employed as a House of Commons cleaner.
Green was arrested at his home last November and held for nine hours by police. A junior law and order official Christopher Galley was also arrested on suspicion of passing the documents over to Green.
Prosecutors later decided there was insufficient evidence to charge either man.
Johnston said in his report that leaks embarrassed the government but "are unlikely to undermine government effectiveness."
The Metropolitan Police's assistant commissioner for specialist operations, John Yates, said the police had been told the leaks from the Home Office may have involved matters relating to national security,
"We accept that some elements of the inquiry could have been carried out differently," he said in a statement.