Anyone with coronavirus symptoms will be ordered to self-isolate for ten days rather than seven as part of new guidelines.
It comes amid fears of a "second wave" sweeping through Europe with rising cases in a number of countries.
At present, anyone showing the main symptoms of Covid-19 must isolate for a week.
But a joint statement from the UK's four Chief Medical Officers has confirmed the change in policy.
Professor Jonathan Van-Tam, England’s deputy chief medical officer, told reporters the change was being made due to the “low but real possibility of infectiousness” between seven and ten days after the onset of the illness.
The Scottish Government has also confirmed its extension to the isolation period.
The statement from the medical officers said: "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."
It follows a warning from UK Health Secretary, Matt Hancock, that a second wave of the virus is "starting to roll across Europe."
Spikes in the figures in Belgium, Luxembourg and Spain have caused particular concern for officials.
Hancock added: “I think you can see a second wave starting to roll across Europe and we’ve got to do everything we can to prevent it from reaching these shores and to tackle it."
