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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 travellers aged 12 and over 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.

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.

Prof 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 the variant 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 hospital admissions 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."

But Dr Katherine Henderson, president of the Royal College of Emergency Medicine, said hospitals were already struggling to cope because of winter pressures, and warned the NHS would be in a "very, very difficult position" if the new variant led to a surge in hospital admissions.

It comes as the UK Health Security Agency reported on Sunday a further 86 cases of the Omicron variant across the UK, taking the total so far to 246.

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.