Some holiday agencies are illegally offering accommodation during the coronavirus outbreak, a council says.
Cornwall Council said it had received complaints that some providers were continuing to host guests, despite the lockdown.
Malcolm Bell, chief executive of Visit Cornwall, said the website Airbnb was still advertising holiday lets.
Airbnb said it was reminding hosts to review government advice "before accepting bookings".
Cornwall Council said it was writing to all holiday accommodation owners and letting platforms in the county as "a small minority" of businesses continued to operate.
It was announced by Boris Johnson that most hotels and similar businesses in the UK must close to guests during the lockdown period.
The council said businesses could face "enforcement action" if they did not obey orders.
It said there were some exemptions allowing businesses to remain open during this period, for example to provide accommodation for key workers, NHS staff and for health and care use.
Mr Bell said: "The truth of the matter is there's no reason to be open for business, in fact it is illegal to be open for business.
"I'm fuming because these are big powerful companies and it must be the same around the world, it's not difficult to know there's a lockdown in Britain, in England, in Cornwall, just get your computer programmers to say no availability."
An Airbnb spokesman said: "On Sunday we announced a platform where hosts on Airbnb are offering NHS and other frontline medical staff free places to stay and Airbnb will waive all fees associated with the stays.
"We are in regular discussions with the government about travel restrictions during the lockdown period and have introduced measures and guidance for both hosts and guests."