The Electoral Commission says it has received a complaint about the election spending of Tory MP Zac Goldsmith.
It was received from The Bureau of Investigative Journalism.
The Electoral Commission declined to comment any further, saying it would follow its usual procedure of carrying out an initial investigation.
Mr Goldsmith, who won the Richmond Park seat in west London at the general election, dismissed the claims as "sleazy journalism".
The Conservatives said: "We are confident that the election expenses in question comply to the spirit and letter of election law."
After its initial investigation, the commission will decide whether it is to carry out an official review of the complaint. This process usually takes about five working days.
A joint investigation by the Bureau of Investigative Journalism, a London-based not-for-profit organisation set up in April, and Channel 4 News raises questions about Mr Goldsmith's spending on campaign materials - such as signs, jackets and leaflets - in the 23 days up to polling.
It queries whether Mr Goldsmith's submitted spending declarations were correct or whether his campaign "reduced the costs in various ways" to remain below the limit.
Mr Goldsmith told the BBC this was "sleazy journalism" of the lowest order, and that he had been targeted because he fought a tight contest in a highly visible seat.
He said he was confident that he had reported all electoral expenditure in line with the formulae used by election agents across the country.
But Channel 4 News said it was standing by its story.
"We refute any suggestion that Zac Goldsmith was targeted simply because he is a high-profile figure," said a spokesman.
"The questions we have raised relating to his expenses are entirely legitimate. The issues we found regarding Mr Goldsmith's campaign expenses are materially different and of a different scale to those found in other returns we looked at."
The Electoral Commission set an £11,003 limit on spending by candidates in Richmond Park in the 23 days between the dissolution of Parliament and the general election on 6 May - known as the short campaign.
Leaflets 'not distributed'
Mr Goldsmith filed a detailed declaration of his spending in June, putting the total £220 below the cap at £10,783.
But after a detailed analysis of his returns, Channel 4 News and the BIJ queried whether he in fact kept his spending below the limit.
Invoices submitted with his returns indicated that Mr Goldsmith spent significantly more on signs, jackets and leaflets than the amounts declared in his spending returns, Channel 4 News claimed.
The Goldsmith team explained that some of the signs were used in different campaigns or were erected before Parliament was dissolved and that many of the leaflets were never distributed. It appears the jackets were not counted as campaign expenditure but stickers attached to them were, according to Channel 4 News.
The MP, a multi-millionaire environmentalist, is the son of the late industrialist Sir James Goldsmith, who founded the Referendum Party, which campaigned for a vote on whether the UK should remain in the European Union.
In 2009, Mr Goldsmith faced controversy over his non-domiciled tax status, which he has since relinquished.
He has previously advised David Cameron on environmental issues.