The hotel which hosted the G7 summit did not have permission to build a bridge allowing world leaders to cross a stream on its beach.
Staff at Carbis Bay Hotel built the temporary structure before the summit without the necessary approval.
That is according to the Environment Agency, who said the venue did not apply for a flood risk permit to divert the stream either.
A spokesperson said: "The Carbis Bay stream is designated as a 'main river' and as such, the Environment Agency have powers to regulate works to this watercourse to ensure flood risk is not increased elsewhere.
"The construction of the temporary bridge would have required our permission via the Flood Risk Activity Permit, prior to the works taking place.
"We can confirm that no Flood Risk Activity Permit application was received and therefore, no permission from us was granted.
"Our flood and coastal risk management enforcement team are aware of these works and will follow up to ensure the removal of any remaining unpermitted structures."
The summit took place at the hotel over the weekend, and was attended by leaders including Prime Minister Boris Johnson, US President Joe Biden and German Chancellor Angela Merkel.
Politicians discussed the environment, economy and the pandemic, and made a commitment to give poorer nations up to one billion Covid doses.
When approached for comment, Carbis Bay Hotel referred the matter to "the G7 media team".
A spokesperson for the G7 Cabinet Office said: "I can confirm that the stream was temporarily diverted and has been returned it its normal course.
"It's worth emphasising that this work only covers where the stream runs over the beach, so obviously its 'course' is affected by daily tides regardless."