An unnamed politician is facing a formal police inquiry into their expenses after a ruling by a special panel.
But the joint Scotland Yard and Crown Prosecution Service (CPS) group formed after the 2009 scandal has decided no action is to be taken against two other parliamentarians whose claims had been called into question.
The CPS refused to identify the individuals involved.
Four ex-MPs and two peers have already been sentenced to prison terms for expenses abuses.
Another former MP, Margaret Moran, has been charged and proceedings are ongoing.
A CPS spokesman said: "The joint panel between the CPS and the Metropolitan Police Service assessing complaints relating to parliamentary expenses was recently considering three individuals.
"As a result, one individual is subject to a criminal investigation by the MPS and two are not.
"The CPS now awaits the outcome of this police investigation and will consider any file of evidence they provide to us."
It is understood the panel now has no more cases to assess.