A spokesman said: "We can confirm that following an assessment of the report published by the Standards and Privileges Committee and following liaison with the Crown Prosecution Service, the Metropolitan Police is reopening the investigation."
Police began examining MacShane's expenses more than two years ago after allegations about abuses surfaced.
The criminal probe was dropped last July. But the ex-Labour minister stepped down from the Commons in November after the committee found he had faked receipts to receive thousands of pounds of public money.
The report included letters from MacShane to the standards commissioner detailing his actions, which had not previously been seen by police.
However, the Commons authorities have suggested such material would not be admissible in court as it is protected by parliamentary privilege.
A CPS spokeswoman said: "On consideration of the report published by the Parliamentary Committee on Standards and Privileges, we have looked again at this case.
"We have continued to liaise with the police in relation to this and have now provided them with further investigative advice."
Parliamentary Standards Commissioner John Lyon found MacShane 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 MacShane himself or someone else "under his authority".
One letter from the MP to Lyon in October 2009 described how he drew funds from the EPI so he could serve on a book-judging panel in Paris.
"I was invited by Jacques Delors to join a committee to draw up a shortlist for the European Book of the Year on which I still serve," he wrote.
"Again, there were no funds to cover the costs of travel and staying in Paris for these meetings and, since I used them to try and advance the case of British writers, including helping to steer the committee to choose the work of British historian (redacted) in 2008, I thought it reasonable to use EPI money to cover these costs."
The cross-party committee said the invoices were "plainly intended to deceive", branding it the "gravest case" they had dealt with.
The parliamentary authorities have confirmed that police were not provided with any of MacShane's evidence or the other information amassed by the commissioner when he referred the case to them in October 2010.
It is understood that officials still believe the correspondence is privileged because it was collected by the commissioner as part of parliamentary proceedings. However, they have suggested the report could be used as a "map" by police to further their own inquiries.