Covid: 'Not practical' to close UK borders, says Boris Johnson
Labour leader Sir Keir Starmer called for tougher restrictions to prevent new coronavirus variants entering the UK.
Boris Johnson said the government had already tightened travel restrictions to reduce the spread of the virus.
He said it would be going ahead with its plan for quarantine hotels for people arriving from Covid hot-spots.
It is not yet clear when the scheme - announced a week ago - will start or when there will be further details.
Speaking during Prime Minister's Questions, Sir Keir said government scientists had recommended "a complete pre-emptive closure of borders".
In response, Mr Johnson said: "Actually, Sage (the Scientific Advisory Group on Emergencies) did not recommend a complete ban and they say travel bans should not be relied upon to stop the importation of new variants.
"But we do have one of the toughest regimes in the world."
Rules around international travel were tightened last month, meaning all travellers must self-isolate for 10 days when they return to the UK.
The government also announced plans to make people quarantine in government-sanctioned hotels when arriving in the UK from certain "high risk" countries, where new coronavirus variants have been identified.
The prime minister said at Wednesday evening's Downing Street briefing that Health Secretary Matt Hancock would make a statement on Thursday to give further details, although No 10 later clarified to say this was not happening.
Labour chair of the Commons Home Affairs Select Committee Yvette Cooper tweeted in response: "Oh for heaven's sake. Who is in charge of this borders policy? When will we get details? Does anyone have a grip?"
The UK has placed 33 high-risk countries on a so-called "red list", including most of South America, southern Africa and Portugal.
Scotland has announced it will extend the measures to all travellers.
Sir Keir said the government's new border arrangements were "still weeks away" from being introduced and would only affect direct flights from some countries.
He asked whether the prime minister was saying that "quarantining all arrivals would make no difference to fighting new variants of the virus", or that it was "too difficult to do it".
Mr Johnson responded by saying that 75% of the UK's medicines come into the country from the European mainland, 45% of food, while 250,000 businesses rely on imports.
"It is not practical to completely close off this country as he seems to be suggesting," he said.