One of the UK government’s scientific advisers has said there must be a “significant reduction” in people coming into the country to combat the South African coronavirus variant, but closing borders completely was impractical.

Prof Calum Semple, who sits on the Scientific Advisory Group for Emergencies (Sage), was speaking after Downing Street denied Boris Johnson had been told by Sage scientists two weeks ago to halt all travel or enforce mandatory hotel quarantines on arrivals from all countries to stop new variants.

After the health secretary, Matt Hancock, announced that cases of the South African variant had been found across England, Labour said it bolstered the argument for closing UK borders or enforcing hotel quarantine for all new arrivals.

Semple, a professor of child health and outbreak medicine at the University of Liverpool, said it was “incredibly important” to “snuff out” the South African variant but there were limits to what could be done.

“It’s much easier if you’re a small island such as the Isle of Man to close the borders,” he told BBC Radio 4’s Today programme.

“Britain is a complex transit country and it’s a much harder decision to make there. But, in general, I do support restricting movement, particularly of people in this time. You can’t do it altogether when you get a country that’s dependent on imports for food and other essential processes, it’s just not practical, but yes a significant reduction in the movement of people is incredibly important at present.”

Semple would not be drawn on whether he would support mandatory hotel quarantines on arrivals from overseas. The government announced last week that UK nationals and residents arriving from locations on the UK travel ban list would have to quarantine at hotels for 10 days. There are 33 countries on the “red list” but Labour says travellers from all countries should be forced to quarantine.

A report in the Times said such an approach, or alternatively a complete travel ban, was recommended by scientists two weeks ago but a senior government source denied that was the case.

Door-to-door testing begins on Tuesday in eight areas of England at the start of a “two-week sprint” to halt the spread of the South African variant. The areas are Hanwell, in west London, Tottenham, in north London, Mitcham, in south London, Walsall, in the West Midland, Broxbourne, in Hertfordshire, Maidstone, in Kent, Woking, in Surrey, and Southport, in Merseyside.

Semple said: “We really just don’t know [the threat posed by the strain],” adding: “But it’s very important to take every opportunity … to catch the so called South African variant, suppress it, keep it down, and allow the vaccine schedule to get ahead of it.”

Prof Robin Shattock, the head of mucosal infection and immunity within the department of medicine at Imperial College London, said he had been working on a vaccine that would target the variant. He told the Today programme a new vaccine could be made in the laboratory in about three weeks and then manufactured in two to three months.