LONDON - Anyone who tests positive or shows symptoms of COVID-19 in Britain will have to self-isolate for 10 days instead of the previous seven, based on a low but tangible possibility that people could remain infectious for longer.

"In symptomatic people COVID-19 is most infectious just before, and for the first few days after symptoms begin," the UK chief medical officers said in a statement on Thursday.

"Evidence, although still limited, has strengthened and shows that people with COVID-19 who are mildly ill and are recovering have a low but real possibility of infectiousness between 7 and 9 days after illness onset."

The government said the change in the rule would come into force immediately.

(Reporting by Paul Sandle; editing by Kate Holton and Estelle Shirbon) ((kate.holton@thomsonreuters.com; 0044 207 542 8560; Reuters Messaging: kate.holton.thomsonreuters.com@reuters.net))