Police are investigating two cases of alleged misuse of taxpayer-funded expenses by MPs.
Expenses watchdog Peter Davis, the Independent Parliamentary Standards Authority's compliance officer, referred three cases to Scotland Yard for assessment, one of which has been finalised and two of which remain with the police.
In his annual report for 2014-15 Mr Davis said he had felt it necessary to refer cases to the police for the first time.
The cases were referred to the police in March, and one has resulted in an MP's aide receiving a caution for fraud, Scotland Yard said.
In his report Mr Davis said: "During the course of the reporting period I have felt it necessary for the first time during my term of office to refer requests for investigation received from Ipsa to the police."
The compliance officer has an agreement with the Metropolitan Police to inform them if there is reason to believe a criminal offence has been committed by an MP or a member of their staff.
Mr Davis said: "Three cases have been forwarded to the Metropolitan Police for assessment; one has been finalised and two remain with the police."
The identities of those involved have not been disclosed.
A Metropolitan Police spokesman said: "Of the three referrals made to us by Ipsa in March 2015, two are being investigated. An assessment of the third referral resulted in a 33-year-old woman, an employee of an MP, receiving a caution in April for fraud by false representation."