People who test positive for coronavirus or show symptoms in the UK must now self-isolate for at least 10 days, rather than seven.
The change, announced by the UK's chief medical officers, comes as ministers try to avoid a resurgence of the virus.
Until now, those showing key symptoms - a new continuous cough, a temperature or loss of taste or smell - have had to self-isolate for at least a week.
The new advice is in line with World Health Organization guidance.