Tourism bosses have advised accommodation providers not to take any bookings from potential visitors living in Tier 3 regions.
They want to prevent people from areas such as Liverpool and Greater Manchester trying to get around lockdown by taking a staycation in the south west, where the infection rate has remained low.
The warning was issued by Cornwall Council and Visit Cornwall.
It follows an online post from one holiday let owner in Devon confirming they had already rejected a visitor from Barnsley, Yorkshire, as it is set to go into Tier 3 by the weekend while urging others to follow their example.
Malcolm Bell, the chief executive of Visit Cornwall, has now advised all people not to take bookings from anyone living in the affected areas.
He also urged people to cancel any planned visits from anyone already booked into accommodation or visiting friends and relatives, unless the stay is one of the exceptions in the Government guidance.
He said: “We all need to play our part in limiting the spread and intensity of the Covid 19 crisis by reinforcing the Government guidance for areas with high level and high restriction, but also that everyone in Cornwall, visitor or residents must double our efforts with hand washing, the wearing of face coverings where required and especially social distancing.”
Cllr Julian German, leader of Cornwall Council, added: “Our first priority is to keep our residents safe during this pandemic and ensure that undue strain is not placed on our health service.
“It is not easy, especially after such a difficult year, to ask any business to turn trade away, but these are exceptional circumstances and we are asking everyone to work with us to keep Cornwall safe.
“In Cornwall we are seeing relatively low numbers of infections, which is welcome. However, we know from other areas of the country how quickly the picture can change.
“I am not willing to undo the fantastic work of our public health team in keeping our residents safe, and that is why I am asking those who visit us to follow the rules. Do not travel to Cornwall if you live in a high-risk area and help us to keep Cornwall safe.
“Our tourism industry is a vital part of the Cornish economy, and we all look forward to the day when we can welcome visitors back with no restrictions.
“However, while we remain in such a precarious position with regards to the pandemic, we must continue to protect our residents.”
The holiday let owner who spoke out was based in Torbay, Devon, which is currently in the lowest category of 'medium'.
In Barnsley, there are 335 coronavirus cases per 100,000, but in Torbay it is just 76.
News of the cancellation was shared on Facebook, and people were quick to share their support.
Sarah Reay wrote: "Absolutely, she did the right thing.
"As a Lancastarian, living in Devon, I would love to visit my friends and family in the north of England, and invite them down here.
"But we are using our common sense, which appears to be lacking in certain areas."
Cathy Edwards said: "It doesn't matter what we think. It's the holiday owner's business - they have to do what they think is right for them."
And Julie Bingham wrote: "Absolutely they’ve done the right thing. For goodness sake, people living in high risk areas should not be travelling anywhere.
"It’s no wonder it’s spreading with people acting like this."