UK nationals and residents returning from 30 "red list" countries will be placed in quarantine in government-provided accommodation - such as hotels - for 10 days, Boris Johnson has told MPs.
The prime minister said the new measures are aimed at preventing new mutant COVID strains from reaching the UK.
He said the government would "enforce this at ports and airports by asking people why they are leaving and instructing them to return home if they do not have a valid reason to travel".
And, announcing the new hotel quarantine requirements, Mr Johnson told MPs the government had already banned travel from countries "where there is a risk of known variants including South Africa, Portugal and South American nations"
"In order to reduce the risk posed by UK nationals and residents returning home from these countries, I can announce that we will require all such arrivals who cannot be refused entry to isolate in government provided accommodation, such as hotels, for 10 days without exception," he added.