health security agency
Transport secretary Mark Harper said one in 45 people in the UK currently has Covid-19.
Other countries, including the US, France and India, have also imposed testing.
The UK Health Security Agency said the measure was “rare”.
Rishi Sunak's spokesman said the NHS was "well prepared" to deal with situations like this.
Much of UK to see heatwave conditions as temperatures expected to rise above 30C this week.
Government urges people to "try back again later" as patience wears thin.
Another study suggests variant is milder but more infectious.
Those infected with the virus can take two lateral flow tests 24 hours apart on day six and seven of their isolation period.
Some 78,610 infections reported as Omicron surges.
The figure was revealed by health secretary Sajid Javid as booster vaccine programme expanded.