Letters in which a former Labour minister admitted expenses abuses cannot be used to prosecute him because they are protected by parliamentary privilege, an official has said.
The report said he submitted 19 false invoices "plainly intended to deceive" Parliament's expenses authority - which said the case was the "gravest" it had dealt with.
There are now calls for a police investigation into Mr MacShane's expense claims, which was dropped in July, to be reopened.
The Metropolitan Police said: "We are aware of the report and will be assessing its content in due course."
The letters, which were never shown to the original inquiry because of parliamentary privilege, are likely to be examined by the police, but are still protected from being used in court.
Clerk of the Journals Liam Laurence Smyth, who is responsible for parliamentary privilege issues, admitted that many people would find the situation "surprising", but said privilege was necessary for Parliament to function effectively.
Even if Mr MacShane had openly admitted criminal behaviour in his evidence, the police would not be able to rely on the comments in court, he said.
However, he suggested the police might now be able to use the letters as a "map" to further their own enquiries.
Conservative MP Philip Davies, who urged the Met to reopen its investigation, said it was a "sad state of affairs" that Mr MacShane was protected by parliamentary privilege.
"All it will do is further undermine the reputation of Parliament," he said.
"There will be millions of people out there who think that MPs are above the law and that is what the perception will be."
Parliamentary Standards Commissioner John Lyon found the MP had entered 19 "misleading" expenses claims for research and translation services from a body called the European Policy Institute (EPI), signed by its supposed general manager.
However, the institute did not exist "in this form" by the time in question and the general manager's signature was provided by Mr MacShane himself or someone else "under his authority".
One letter from the MP to Mr Lyon in October 2009 described how he drew funds from the EPI so he could serve on a book-judging panel in Paris.
"I appreciate the committee's ruling that I made no personal gain and I regret my foolishness in the manner I chose to be reimbursed for work including working as the Prime Minister's personal envoy in Europe," he said.