Airbnb hosts advertising properties as coronavirus hideouts are "dangerous and irresponsible", the government says.
Owners have listed homes as being "Covid-19 retreats" and "perfect for isolating with family" in the British countryside.
New coronavirus laws say holiday accommodation should be provided only to keyworkers needing to self-isolate.
But some listings on the site allow instant booking of rentals without any vetting, BBC News discovered.
The properties advertised as places to self-isolate include an "idyllic cottage", a houseboat and even a castle.
Only one Airbnb host contacted by BBC News said their rental was available solely to keyworkers.
But others complained the lockdown had disrupted their business.
In response to the BBC News's findings, Tourism Minister Nigel Huddleston said: "Our advice is clear.
"Essential travel does not include holidays, leisure travel and visits to second homes - and people must remain in their primary residence.
“It is incredibly irresponsible, and dangerous for some property owners to be marketing themselves as ‘isolation retreats’.
"We are writing to companies today to remind them of their responsibilities at this time."
Individuals and businesses could face fines of up to £960 for breaking these rules, the government added.
Last month, Airbnb announced a worldwide extension to its “extenuating circumstances” policy, stating all guests booked for check-ins between 14 March and 31 May would be eligible for full-refund cancellations.
In a message on its website, the company acknowledged the decision to offer guests a refund had caused hardship for many hosts and it would pay £200m to help cover the cost of these cancellations.
But one Airbnb host told BBC News they had received no clear instructions from the platform to say they were not allowed to take reservations.
The US-based company has yet to respond to the government's intervention.
But earlier this month it released a statement saying: "We announced a platform where hosts on Airbnb are offering NHS and other front-line 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."