Nearly 500 people with coronavirus are thought to have arrived in UK airports in April, a top scientific adviser to the Government has said.
Boris Johnson announced on Sunday that Britain would “impose quarantine on people coming into this country by air” when the domestic infection rate had lowered.
No start or end date has been set for the measures, which exempt passengers from Ireland.
So far during the pandemic, no travellers have been required to self-isolate or take tests after landing in the UK.
But it has now emerged that up to 475 infected people are thought to have arrived in airports in April alone.
Professor John Aston told the Commons' Science and Technology Committee on Wednesday that 95,000 people arrived in the UK by plane between April 1 and 26, including 53,000 UK citizens.
Asked if there was an estimate of how many of those had Covid-19, he replied: "We believe that less than 0.5 per cent of those people arriving potentially had Covid-19."
Prof Aston told the committee that the Scientific Advisory Group for Emergencies (Sage) used "complicated modelling" to calculate the figure.
"It requires you to understand the prevalence (of Covid-19) within overseas countries as well as the prevalence within the UK," he said.
The Government confirmed the 14-day quarantine would be enforced after publishing its “roadmap” out of lockdown on Monday.
It comes as Downing Street has strongly denied reports that travellers from France would be exempt from the restrictions.
The Prime Minister’s spokesman added that there was a “particular issue” around the movement of freight, said Number 10, but added that “our plans for the rest of the world, we will set those out in due course”.
Aviation chiefs have raised concern over the move and are calling for an international standard that applies to borders globally.
