Mouthwashes with at least 0.07 per cent cetylpyridinium chloride show "promising signs" of being able to combat the virus, the Cardiff university study says.
Dr Nick Claydon, a specialist periodontologist, said he believed mouthwash can become an important addition to fighting coronavirus along with "handwashing, physical distancing and wearing masks, both now and in the future".
Researches who conducted the test in the lab setting used mouthwash brands such as Dentyl.
Dentyl is the only United Kingdom mouthwash brand to take part in the 12-week clinical trial, which is led by Professor David Thomas from Cardiff University and titled: "The measurement of mouthwash anti-viral activity against Covid-19".
"This study is not yet peer reviewed and published which means it has not yet been scrutinised by other scientists as is the usual process with academic research".
"People should continue to follow the preventive measures issued by the United Kingdom government, including washing hands frequently and maintaining social distance".
The preliminary result comes ahead of a clinical trial into whether using over-the-counter mouthwash has the potential to reduce the levels of Covid-19 in a patient's saliva.
Prof David Thomas, from the university, said the initial results were encouraging, but the clinical trial would not produce evidence of how to prevent transmission between patients.
The scientists leading the study said they now need to see if the mouthwash effectively eradicates the virus in patients.
"We need to understand if the effect of over-the-counter mouthwashes on the Covid-19 virus achieved in the laboratory can be reproduced in patients, and we look forward to completing our clinical trial in early 2021".