The Cambridge Institute of Therapeutic Immunology and Infectious Disease study concluded mutations on the virus's spike protein, which makes it able to avoid antibodies, may also reduce how it replicates in the lungs and causes severe disease.

"These observations highlight that Omicron has gained immune evasion properties whilst compromising on properties associated with replication and pathogenicity," the study's abstract stated.

Study leader Ravi Gupta, a member of the Scientific Advisory Group for Emergencies and professor of clinical microbiology at Cambridge, said there are still challenges ahead despite the seemingly positive findings

He tweeted: "What does this all mean? Efficient infection of lung cells could correlate with severity of lung disease.

"Syncytia or fused cells are often seen in respiratory tissues taken following severe disease. Delta was very good at both, in contrast to Omicron. Further work is needed.

"In summary this work suggests that Omicron does appear to have become more immune evasive, but that properties associated with disease progression *may* be attenuated to some extent. The significant growth of Omicron nevertheless represents a major public health challenge."

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The latest figures from the UK Health Security Agency this weekend showed there have been 10,059 additional confirmed cases of the Omicron variant of COVID reported across the UK.

The findings come after the Scientific Advisory Group for Emergencies (Sage) warned it is "almost certain that there are now hundreds of thousands of new Omicron infections per day" in England.

This has prompted speculation that tougher pandemic restrictions could be introduced after Christmas.

London Mayor Sadiq Khan also declared a major incident yesterday, due to the "hugely concerning" surge in cases across the capital.

He also expressed fears about staff absences in crucial public services including the NHS, the fire service and police due to infection from COVID.

Mr Khan said: "The surge in cases of the Omicron variant across our capital is hugely concerning, so we are once again declaring a major incident because of the threat of COVID to our city.

"The Omicron variant has quickly become dominant, with cases increasing rapidly and the number of patients in our hospitals with COVID on the rise again."

When asked about a possible tightening of restrictions after Christmas, a government spokesman said they will "continue to look closely at all data and keep our measures under review".