It is "too late" to effectively halt the spread of the omicron variant in the UK, a government science adviser has warned.
On Saturday, it was announced all travellers arriving in England will be required to take a Covid-19 pre-departure test from Tuesday - while Nigeria is being added to the Government's travel red list.
Ministers said the extra test was intended to be a temporary measure following new data showing an increase in the number of cases of the new strain linked to foreign travel.
But Prof Mark Woolhouse, who is a member of the Scientific Pandemic Influenza Group on Modelling (Spi-M) which advises the Government, said the measures would not make a "material difference" as the variant is already "spreading pretty rapidly".
He told BBC One's Andrew Marr Show on Sunday: "I think that may be a case of shutting the stable door after the horse has bolted.
"If omicron is here in the UK, and it certainly is, if there's community transmission in the UK, and it certainly looks that way, then it's that community transmission that will drive a next wave.
"The cases that are being imported are important, we want to detect those and isolate any positive cases we find, as we would for any case anywhere.
"But I think it's too late to make a material difference to the course of the omicron wave, if we're going to have one."
The travel industry reacted with fury after the latest measures were announced, despite ministers insisting they were only "temporary".
Dominic Raab defends new travel measures
Deputy Prime Minister Dominic Raab told Sky News's Trevor Phillips On Sunday programme: "I know that is a burden for the travel industry but we have made huge, huge strides in this country.
"We have got to take the measures targeted forensically to stop the new variant seeding in this country to create a bigger problem.
"We have taken a balanced approach but we are always alert to extra risk that takes us back not forward."
Prof Woolhouse said although the numbers of people with the omicron variant are "still quite small" and likely remain in the hundreds, they are "growing quite fast".
However, he insisted that vaccinations will still be "very, very good" at protecting against the new variant.
Statistician Prof Sir David Spiegelhalter told Sky News's Trevor Phillips On Sunday programme: "It's a very difficult situation because we haven't got a lot of data yet at all, almost nothing from this country about what the risks are.
"In South Africa there's data coming out showing pretty strong evidence that the increased risk of transmission and some evidence about people going to hospital, but it may actually be milder but we haven't got enough data yet to be able to say.
"It doesn't look as if it's really severe if you get it, I think that's about all we can say at the moment."
Asked if measures to combat the spread of omicron have gone far enough, Prof Sir David added: "It's best to be precautionary, when there's so much we don't know... and when we don't know it's better to be safe than sorry."
Omicron 'highly transmissible' but causes 'milder' disease
A South African health researcher said early data suggests omicron is highly transmissible, but has a less than 1% per cent chance of re-infection and typically results in "milder" disease.
Speaking on The Andrew Marr Show, Professor Willem Hanekom, director of the Africa Health Research Institute in South Africa said: "We know three things that we didn't know last week, the first thing is that the virus is spreading extraordinarily fast in South Africa, the increase in cases is much steeper than it's been in the past three waves so it seems that omicron is able to spread very easily and virtually all the cases that we see in South Africa right now are omicron.
"The second thing we have data on is re-infections, so as you know, after you've had Covid you have about a 1 per cent chance, or perhaps even less than a 1 per cent chance, of getting re-infected and even getting disease again, of course, by this virus.
"The third little bit of data we have already relates to clinical cases and how severe the disease is. The only data suggests the disease may occur more in younger people and mostly younger people who are unvaccinated and overall so far the disease has appeared to be milder but again I want to say we have to be cautious - these are very early days."