The Prime Minister insisted the UK was the first country to take “decisive measures” in relation to the new strain.
From Tuesday all travellers arriving in England will be required to take a Covid-19 pre-departure test in an attempt to limit the spread of the Omicron variant.
Nigeria was added to the red list at 4am on Monday, and arrivals from the African country must now spend 11 nights in a quarantine hotel.
There are 11 destinations on the list.
A requirement for people entering the UK to self-isolate until they receive a negative result from a post-arrival PCR test – even if they are fully vaccinated – was introduced last week.
Professor Mark Woolhouse, who is a member of the Scientific Pandemic Influenza Group on Modelling (SPI-M) which advises the Government, said the Omicron variant is already “spreading pretty rapidly” and the new travel measures “may be a case of shutting the stable door after the horse has bolted”.
On a visit to Merseyside on Monday, Mr Johnson told reporters: “What we’re doing is responding to the pandemic.
“We were the first country in the world to take decisive measures to tackle Omicron. We put about 10 countries automatically, immediately, on to the red list and we said that anybody coming from any country in the world would have to quarantine for a couple of days.
“We’re now going further and toughening those measures up as we see the spread of Omicron around the world.”
The Prime Minister added that the Government does not believe it needs to change the “overall guidance and advice we’re giving about Omicron in this country”.
He added: “We’re still waiting to see exactly how dangerous it is, what sort of effect it has in terms of deaths and hospitalisations.”Internet Explorer Channel Network