Millionaire MP Zac Goldsmith will not be reported to police over his election spending.
But the Electoral Commission said it was concerned about the way in which the Conservative MP's team filled out spending returns for his successful bid to win the Richmond Park seat in the May general election.
Mr Goldsmith may have under-reported his spending in the final weeks before polling day by at least 1,185, which would have resulted in an overspend of 966 during this period, it said. However, the Commission found that even if this were the case, his overall spending came in well under the 39,856 total permitted.
The return submitted by Mr Goldsmith's election agent David Newman was "unclear in places" and the way in which some costs were shared between the general election campaign and a concurrent local council contest was "not consistent with the Commission's guidance or good practice".
But the watchdog found no evidence of intentional rule-breaking and decided it would be inappropriate to refer the case to the police for investigation.
Mr Goldsmith said: "We are delighted to have been vindicated as we always knew we would be. There is no doubt that the rules are horribly complicated and it may well be that the Electoral Commission should look to clarifying them in the interests of every parliamentary candidate."