Its creators aim to deliver insights into why some people get the disease more severely than others.
They also hope to create a map showing where outbreaks are happening and help distinguish cases from those of the common cold.
It is one of many such new apps. Experts have warned people to be cautious about which they download.
At present, COVID Symptom Tracker is the third most popular app in Apple’s UK store and second in Google Play’s new releases chart for the country.
Its developers are targeting one million downloads in 24 hours.
The aim of the app is to track in real-time how the disease progresses to help slow the outbreak and to identify: How fast the virus is spreading in your area, a high-risk area in the country, and who is most at risk, by better understanding symptoms linked to underlying health conditions.
Researchers believe that the data from the study will reveal important information about the symptoms and progress of the COVID-19 infection in different people, and why some go on develop the more severe or fatal disease while others have only mild symptoms.
The program has been shared thousands of times via WhatsApp and other social media platforms.
Created in just three days by researchers at Guy’s and St Thomas’ hospitals and King’s College London university, in conjunction with the nutrition advice start-up Zoe, it has already reached 750,000 downloads and, according to its developers, is being installed at a rate of 50,000 times an hour.
The app was the brainchild of Prof Tim Spector, a genetic epidemiologist at King’s College, who has specialized in the genetics and medical histories of twins for the past 25 years.
Initially, the app was made available only to the twins taking part in his studies, who were sent a home-testing kit to better understand which symptoms corresponded to the coronavirus.
The professor realized it could be scaled up to the general public, without the testing element.
"There are lots of scams out there and bogus things trying to get your details," he said.
People wishing to download the COVID-19 tracker can do so from Apple and Google's app stores.