The latest travel rule changes are “a case of shutting the stable door after the horse has bolted,” a scientist advising the government has said.
Prof Mark Woolhouse said the new rules had come “too late” to make a “material difference” to a potential wave of the Omicron variant in the UK.
The changes include pre-departure tests for people arriving in the UK and Nigeria going on the travel red list.
No 10 said the changes were due to a rise in travel-linked Omicron cases.
From 04:00 GMT on Tuesday everyone aged 12 and over will have to take a test a maximum of 48 hours before leaving.
Under the changes, passengers will be required to show proof of a negative PCR or lateral flow test taken no earlier than 48 hours before departure.
Health Secretary Sajid Javid said the measures were temporary.
“Public safety is a priority, but businesses will fail, travellers will be stranded and livelihoods devastated by the lack of coherent plans from government.”
Nigeria will also be added to the travel red list of countries from Monday, which means UK or Irish nationals, or UK residents returning from the country must quarantine in a hotel for 10 days.
But the latest changes have been described by the travel industry as a “hammer blow”, with the Business Travel Association warning livelihoods would be “devastated”.
Deputy Prime Minister Dominic Raab accepted the latest measures might “upset or disrupt” some people, but insisted it was right for ministers to take “incremental steps” early to avoid “bigger disruption” to travel and the economy.
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Prof Mark Woolhouse, a member of the government’s Scientific Pandemic Influenza Group on Modelling (Spi-M), told the BBC’s Andrew Marr that while imported cases of the Omicron variant were “important”, community transmission of Omicron would be the driver of a next wave.
He added the numbers of UK Omicron cases were currently “quite small” and he strongly suspected the absolute number was “more in the hundreds than the thousands”.
But he warned Omicron was “spreading pretty rapidly” in the UK, and if current trends here and in South Africa continued in the coming weeks and months, it could even replace the Delta variant around the world.
Asked about Prof Woolhouse’s comment the new restrictions had come too late, Mr Raab said there would always be “Goldilocks criticism that we’ve done too much or we’ve done too little”.
He said ministers would stay “eternally vigilant” to the risks associated with the Omicron variant and that measures, including adding countries to the travel red list, were intended to prevent its “reseeding” in the UK.
Citing this week’s drop in the number of Covid hospitalisations and deaths and the “success” of the vaccination rollout, he said: “We have steadily and steadfastly created the resilient defences which means we can enjoy Christmas.”
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It comes as the latest figures from the UK Health Security Agency on Saturday showed a further 26 cases of the Omicron variant had been reported across the UK – taking the total so far to 160.
The Department of Health and Social Care said 21 UK Omicron cases linked to Nigerian travel had been discovered in recent days.
Mr Javid said this number was growing and Nigeria was now second only to South Africa in terms of linked cases to Omicron.
Scientists have raised concerns the heavily-mutated variant may be more transmissible than the dominant Delta strain and be able to escape immunity from vaccines.
Ten southern African countries had already been added to the UK’s travel red list, because of fears about Omicron – South Africa, Namibia, Zimbabwe, Botswana, Lesotho, Eswatini, Angola, Mozambique, Malawi and Zambia.
‘It’s already here’
However, travel industry bosses said they had not been given sufficient warning of the travel rule changes.
Clive Wratten, chief executive of the Business Travel Association, said: “The introduction of pre-departure testing with little warning is a hammer blow to the business travel industry.”
“Public safety is a priority, but businesses will fail, travellers will be stranded and livelihoods devastated by the lack of coherent plans from government,” he added.
Airlines UK said the change was a “premature” move that would “hit industry and passengers before we see the full data” on the effect of the Omicron variant.
The trade body added that previous experiences showed “blanket restrictions do not stop the importation of variants”, adding the variant was “already here”.
Travel association Abta called for the government “to step up to save jobs and businesses” and for the cost of PCR tests to be reduced.
Currently, travellers need to self-isolate until they test negative within two days of arriving into the country.
The government has said the change to the rules followed new analysis from the UK Health and Security Agency that indicated the window between infection and infectiousness may be shorter for the Omicron variant.
This increases the effectiveness of pre-departure testing as it is more likely to identify positive cases before travel, the government added.
The Labour party had called for pre-departure tests to be brought in, with Shadow Home Secretary Yvette Cooper saying she was “relieved” it was now happening, but wished ministers had acted faster.
The Scottish and Welsh governments have confirmed they will also bring in the updated measures for travellers following the assessment of the UK Health Security Agency.
Last week the government’s scientific advisory body Sage said pre-departure tests for those arriving in the UK would be valuable, in the leaked minutes of a meeting seen by the BBC.