A firm planning to extract tin from the seabed off Cornwall has broken a coastal protection law, a council said.
Cornwall Council has told Marine Minerals Limited (MML) that it broke the Coast Protection Act by not getting permission to get samples.
But the authority said it would not be prosecuting the firm because it had not been asked to get permission.
MML said it had gone to "great lengths" to get the necessary permissions and it was a "genuine mistake".
MML wants to extract tin washed down from former tin mines and has taken five tonnes of seabed material for sampling.
It estimates that there are about 22,000 tonnes of tin on the seabed worth "tens of millions of pounds".
Cornwall Council said in a statement that MML had contacted the Marine Management Organisation (MMO) about getting samples.
The council said: "Unfortunately the company was not then advised to apply to the council as the relevant authority for a licence to carry out these works.
"Although this was a breach of the Coast Protection Act, the council has decided that a prosecution would not be appropriate in these circumstances.
"However the authority is writing to the company advising them of their obligations if they wish to carry out further sampling runs, or a larger scale extraction of materials from the sea bed."
Chris Davies of MML said: "It was a genuine mistake.
"Of course we will fully comply with all the relevant regulations in the future.
"If it was an oversight it was an oversight by all of us."
Campaigners Surfers Against Sewage (SAS) said it had warned MML about the Coast Protection Act before it started sampling.