They also warn fraudsters could start to target people using phishing emails related to the coronavirus.
Scammers posing as coronavirus testers are trying to “cash in” on the crisis by targeting vulnerable people, police have warned.
South London's Lambeth force said they had reports that "individuals may be taking advantage of the vulnerable by posing as door-to-door coronavirus testers in order to gain access to people's properties".
They said people should call them on 999 if anyone comes to their door and claims to be conducting virus tests.
Neither the police nor the NHS are conducting such tests.
"Unfortunately there are some people who will try to cash in on people's vulnerabilities at a time when they may be distracted by concerns regarding the coronavirus," said a Metropolitan Police briefing note.
Residents in Isleworth, west London, were also warned by local officers about the door-to-door testing scam.
The official police advice is: "If someone knocks at your door claiming to be from a company or organisation, first check their ID.
"If you're not happy, don't let them in. Ask the person to wait outside, shut the door and contact the organisation by using a trusted method. If they're genuine, they'll understand."
Police also warned that scammers may eventually try to use the crisis to trick people into revealing personal and financial details using "coronavirus-themed phishing emails" and "malicious attachments".
The number of people who have died after contracting coronavirus in England rose to 128 in the latest figures - an increase of 29 in 24 hours.
Source: BBC News