Coronavirus: Virus isolation period extended from seven to 10 days
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.
The chief medical officers said the change 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".
Evidence shows that people with Covid-19 have "a low but real possibility of infectiousness" between seven and nine days after falling ill, they said.
It comes after the prime minister warned of signs of a "second wave" of the pandemic in parts of Europe.
Speaking on BBC Breakfast before the announcement, Health Secretary Matt Hancock said the government wanted to "take a precautionary approach" and "protect people from that wave reaching our shores".
There have also been concerns about several local outbreaks across the UK, including in Oldham, Wrexham and Staffordshire.
Meanwhile, the government is also expected on Thursday to review the restrictions in Leicester again, a month after the city was put into extended lockdown following a spike in cases.
Analysis by Nick Triggle, BBC Health correspondent
The UK requirement for people who test positive or show symptoms to isolate for seven days was always on the low side - plenty of other countries require 10 days or even longer.
As coronavirus is new, there is still a lack of certainty over how long someone remains infectious, which is why there are a variety of approaches.
Research suggests the most infectious period is just before symptoms start and for the first few days afterwards - that's what makes it such a difficult virus to contain.
But UK health officials say there is enough evidence now to point to a "low, but tangible" risk of transmission from seven to 10 days, even for those with mild to moderate illness.
This is a decision that could have been taken weeks, even months, ago in truth.
But officials are describing it as a "value judgement" based on a variety of reasons.
The availability of widespread testing means only those with a positively confirmed infection need to continue isolating - earlier in the pandemic the lack of testing meant a blanket approach had to be taken so anyone with symptoms was asked to isolate.
It is also about sending an important signal. Levels of infection are low, but the message is clear: we can't be complacent.
Prof Peter Openshaw, who is part of a body advising the government on respiratory viruses, told the BBC's Today programme that most transmission takes place "at the time of symptoms developing, possibly for two days before".
However, he said 10 days would be a "safer margin" because there have been "rare cases" in which people have been able to pass the virus on for up to nine days.
"We don't really know that there's very much transmission actually going on in those last couple of days, but I think in terms of trying to generally put pressure on the virus now… I can absolutely understand why the government might wish to introduce these changes," he said.
Those returning to the UK from certain countries are also being asked to quarantine for 14 days - a move that has sparked complaints from travel firms.
Ministers are also looking for a way to reduce the current 14-day quarantine period for arrivals to the UK, meaning that quarantine and self-isolation time periods could be standardised at 10 days.
Mr Hancock told Today that the government is looking at using testing to reduce the length of the quarantine period - but there would be no imminent changes.
He did, however, say that new countries could be added to the quarantine list in the coming days.
On Wednesday evening the government held a meeting to discuss which other countries would be joining the list of those not exempt from the quarantine rule, after a spike in cases saw Spain added over the weekend.
A senior government source told the BBC that Luxembourg was likely to be added this week but that - for the moment - the numbers in Belgium would continue to be monitored.
On Wednesday, leading travel and airline industry figures called for coronavirus testing at airports in the UK but the source said that "any new system had to be workable and at the moment it isn't".
Mr Hancock told Breakfast that the "big scientific challenge" with testing arrivals at the border is that people "can incubate this disease for many days without displaying any symptoms, and that wouldn't show up in a test".
Officials said a further 83 people with coronavirus had died in the UK, taking the total number of deaths to 45,961.
It also reported another 763 confirmed cases, taking the total to 301,455.
What are the rules on self-isolating?
Self-isolating means staying at home and not leaving it.
Anyone who has either tested positive for coronavirus or shown symptoms should isolate themselves for at least 10 days, and those who show symptoms should arrange to get tested.
a new continuous cough
a high temperature
a loss of, or change in, your normal sense of taste or smell
They should continue to self-isolate if they still feel unwell after that initial period.
Other members of their household should isolate for 14 days and not leave their homes.
If you test positive you will be contacted by contact tracers, who will establish who else you might have passed on the infection to.
Anybody they deem to be at risk will have to isolate themselves for 14 days from the point of contact.