A new COVID variant with a "high number of mutations" has been detected in the UK.
The mutation, known as BA.2.86, was identified in the UK on Friday in an individual with no recent travel history, the UK Health and Security Agency (UKHSA) said.
This means there could already be "significant community transmission" among Britons, the agency said.
The "high number of mutations" - 33 to be precise - means that spike proteins, the membranes on the outside of the virus that allow it to enter and infect human cells, will change their shape.
"Having changed their shape, they may become more infectious, they may become more disease-causing," Dr Bharat Pankhania, an infectious disease control expert from the University of Exeter, told Sky News.
"On the other hand, they may not. We just don't know yet," he said.
The BA.2.86 mutation was first detected in Denmark on 24 July and has also been discovered in Israel and the US.
It is thought to be the likely ancestor of the BA.2 variant, nicknamed "stealth Omicron", which originated in southern Africa and was first detected in the UK by late 2021.
It comes after a variant known as EG.5.1 was reported to be making up one in seven new cases in the UK.
The UKHSA said there is "insufficient data" to assess how serious the BA.2.86 strain might be, or how likely it is that current vaccines will protect against it.
It is "the most striking SARS-CoV-2 strain the world has witnessed since the emergence of Omicron", Francois Balloux, professor of computational systems biology and director of the UCL Genetics Institute at University College London, said.
However, it is unlikely to cause a fresh wave of severe disease and deaths, or prompt fresh restrictions on people's daily lives, because most people have some immunity to the illness.
"Even if people get reinfected by BA.2.86, immune memory will still allow their immune system to kick in and control the infection far more effectively," Prof Balloux said.
"It remains that a large wave of infection by BA.2.86, or any future comparable variant, would be an unwelcome event."
It is likely to have emerged in an immunocompromised person - someone who has a weaker immune system - who later spread it, Prof Balloux said.
He said that global vaccination is the best thing to do to reduce infections.
The UKHSA said it is "undertaking detailed assessment" and will provide further information on the new variant in due course.