The Electoral Commission is to review the election spending of Tory MP Zac Goldsmith after receiving a complaint.
The Bureau of Investigative Journalism raised questions about his spending on campaign materials and whether submitted declarations were correct.
The Commission said it would seek to "establish the facts of the matter" and this did not imply any judgement.
The MP welcomed the review as it would find he was "absolutely scrupulous" in complying with the rules.
After examining evidence submitted by the BIJ, a London-based not-for-profit organisation set up in April, the watchdog said it was carrying out a case review.
This falls short of a fully-fledged investigation but could lead to a ruling over whether there has been a breach of election law.
Mr Goldsmith, who won the Richmond Park seat in west London at the general election, has dismissed the claims as "sleazy journalism".
The Conservatives have said they are "confident" that the MP's election expenses comply with "the spirit and letter of election law".
The Commission said the review would take a maximum of 90 days.