People in the UK who have tested positive for COVID-19 or present symptoms must now self-isolate for 10 days instead of seven.
"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 to 9 days after illness onset," the UK's Chief Medical Officers said in a statement.
They added that given "widespread and rapid testing" is now available and that lockdown measures have been reduced, the extension "will help provide additional protection to others in the community".
"This is particularly important to protect those who have been shielding and in advance of the autumn and winter when we may see increased community transmission," they went on.
Varying quarantine period
Britain's updated guidance is in line with France, where the self-isolation period for people who test positive is set at eight days plus two days without symptoms — a total of 10 days.
The World Health Organisation (WHO) has been advising since late May that asymptomatic people should self-isolate for 10 days after testing positive but recommends that people presenting symptoms self-isolate for 10 days plus at least three days without symptoms.
Second wave fears
The self-isolation extension comes as Health Secretary Matt Hancock warned that "we can see, sadly, a second wave of coronavirus that's starting to roll across Europe".
He told the BBC that the government thus aimed to take a "precautionary approach" and "do everything possible to protect people from that wave reaching our shores."
The UK is Europe's most heavily-impacted country with more than 46,000 fatalities recorded since the beginning of the pandemic, according to a tally by Johns Hopkins University.
Hancock has launched an urgent review into the way Public Health England tallies up its COVID-19 figures after it emerged that people who had once tested positive but later died of other causes were included in the death toll.
He stressed last week in front of a parliamentary committee that excess mortality figures — which calculates the difference in the number of deaths over a similar period of time across multiple years — will provide the "only true measure of the impact of this in terms of mortality."
Figures released on Thursday by the Office for National Statistics showed that England had the highest levels of excess mortality in Europe between January and mid-June.
"While England did not have the highest peak mortality, it did have the longest continuous period of excess mortality of any country compared, resulting in England having the highest levels of excess mortality in Europe for the period as a whole," the ONS report states.