Professor Babak Javid, a consultant in infectious diseases at Cambridge University, told BBC Radio 4's Today programme that a low level of oxygen in the blood was a symptom of coronavirus and could be detected using a pulse oximeter.
He said oxygen levels that dip below 96 per cent are a "danger sign", particularly when they occur after "mild exercise, such as walking up the stairs or going for a short walk".
He added: "If it dips at that time, that's a real warning signal. One of the things I would say is not to be a doctor at home, as it were, but if you have one of these machines and you are concerned, to call your GP practice because certainly the NHS is willing to see people earlier than they are at the moment."
Pulse oximeters are commonly used by GPs as part of check-ups and clip to the finger or ear to provide a reading. They are currently available online for around £20 to £40, but retailers appear either to have low stock levels or long waits for delivery.
One listing on Amazon said the item would not be delivered until June.
Reports in the US said there has been a spike in sales of the devices, with pharmacies Walgreens and CVS reportedly selling out last week.
Deborah Corbett, the managing director of Core Medical Ltd, a UK-based manufacturer of first aid kits, said she had seen a similar rise in interest in Britain. The company is testing three models to ensure they are accurate.
"There's definitely been an incline in the market for pulse oximeters, but the same is true for anything to do with Covid-19," she said.
"The Government is working with scientists and doctors to try and get as many ways as possible of combating this terrible virus."
Her "big reservation" about the devices is ensuring they work, and her firm is undertaking "thorough testing" before supplying them.
The NHS has previously warned that a large number of factors could create a false reading, including ambient light, cold fingers, poor circulation and wearing nail varnish.
A healthy person at sea level should have a saturation of 95 per cent or more, according to one piece of local NHS guidance.
In 2018, the health service said devices meant to be used on the finger but attached to the ear or those meant for use by adults but administered to children can produce results "50 per cent lower or 30 per cent higher" than the true value.
The devices cannot tell someone if they have coronavirus, but could give an indication that further checks may be necessary.
Dr Dan Bunstone, the chief medical officer for online GP service Push Doctor, said: "The recommendation for individuals to use an oximeter at home should be issued with caution.
"These devices come with limitations if not used correctly, producing inaccurate readings and potentially causing unnecessary worry."
He also warned that a rush on the devices could cause "resource depletion" for the NHS.
