London could see tens of thousands of measles cases due to low levels of vaccination, the UK Health Security Agency (UKHSA) has warned.
Mathematical calculations suggest an outbreak could affect between 40,000 and 160,000 people.
Higher levels of immunisation in the rest of the UK means there is a "low risk" of a large epidemic elsewhere.
But the UKHSA said there was an "urgent" need to vaccinate children, teenagers and young adults.
Levels of measles, mumps and rubella (MMR) vaccinations in the UK are at their lowest level in a decade, with around one in ten children not protected by the time they start primary school.
Immunisations also took a significant dip in the early 2000s after claims of a link between the MMR jab and autism. This has been completely discredited and the doctor who pushed the idea, Andrew Wakefield, was struck off the medical register.
But measles is one of the most contagious diseases around, and growing numbers of people are without protection. Scientists at the UKHSA and the London School of Hygiene and Tropical Medicine have run the numbers to reach those predictions of tens of thousands of cases in the capital.
This is a theoretical risk, rather than saying we are already at the start of a huge measles outbreak. There have been 128 cases so far this year, compared with 54 in the whole of 2022.
But the latest risk assessment suggests the R number – if you remember from Covid that's the number of people an infected person gives the virus to – has exceeded, or is close to 1.0, which is the point where a virus can take off.
The assessment also highlights 19 to 25-year-olds – who would have missed out at the peak of the autism scare – as being the most susceptible. There is heightened concern around university students.
The UKHSA also says a large outbreak could put pressure on the NHS, with between 20% and 40% of infected people needing hospital care.
"Due to longstanding sub-optimal vaccine uptake there is now a very real risk of seeing big outbreaks in London," said Dr Vanessa Saliba, a consultant epidemiologist at the UKHSA.
"Nobody wants to see their child or loved ones sick with measles, or put others who are more vulnerable, like babies, at risk. I urge those who have missed their MMR vaccines to catch up now."
Measles vaccination rates have taken a knock around the world, as the Covid pandemic has disrupted normal healthcare.
Outbreaks in South Asia and Africa also increase the risk of the virus being brought into the UK, where it can then take off.
The NHS is launching catch-up programmes, and parents are encouraged to check their children are up-to-date with their vaccines.
Measles usually starts as a cold and then is followed by a rash – but there can be serious complications for some.
"For every 1,000 children who get measles, one or two will die from it," said Prof Beate Kampmann, professor of paediatric infection and immunity at London School of Hygiene and Tropical Medicine.
"To avoid deaths, serious cases and a community outbreak, 95% of the population needs to have been vaccinated against this infection, but our current coverage is well below this target."
Children in the UK should have a first dose of the MMR vaccine by their first birthday and the second dose by the time they are three-and-a-half years old.
This protects them for life against measles and prevents spread of the virus to other people.
Jane Clegg, Regional Chief Nurse for the NHS in London said: "If you have any questions or concerns, please get in touch with your GP practice or local pharmacist for advice. Now's the time to act to protect yourself and your loved ones from measles."
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MMR (measles, mumps and rubella) vaccine – NHS
Measles – NHS
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