A "big measles outbreak" in parts of London is affecting unvaccinated children under the age of 10, the UK Health Security Agency (UKHSA) has confirmed.
It said the outbreak in schools and nurseries in north-east London had left some children requiring hospital treatment.
The agency had previously reported 34 laboratory-confirmed measles cases in Enfield from January 1 to February 9.
More than 60 suspected cases of measles have been reported by seven schools and a nursery in Enfield, the Sunday Times reports.
Measles is a highly infectious viral illness that can spread very easily among people who are not fully vaccinated.
Our latest data shows we are now seeing a big measles outbreak in north-east London, mostly affecting unvaccinated children under 10
Dr Vanessa Saliba, UKHSA
While many people recover, the illness can lead to serious complications such as pneumonia, brain inflammation and, in rare cases, long-term disability or death.
Dr Vanessa Saliba, consultant epidemiologist at the UKHSA, said: "Our latest data shows we are now seeing a big measles outbreak in north-east London, mostly affecting unvaccinated children under 10 in schools and nurseries, with some being hospitalised.
"Measles is a nasty illness for any child, but for some it can lead to long-term complications and tragically death, but is so easily preventable with two doses of the MMRV vaccine.
"If your child has missed any of their doses it's important to catch up as soon as possible, giving them vital protection against this highly contagious disease, but also helping to protect more vulnerable children around them who are too young or unable to have the vaccine due to a health condition.
"With Easter holidays fast approaching, it is a timely reminder to families travelling overseas to ensure all family members, especially children, are vaccinated, as measles is widespread in some countries with close links to the UK and there are ongoing outbreaks in parts of Europe."
Global health officials announced earlier this year the UK is no longer considered to have eliminated measles.
Figures published last August by the UKHSA showed just 64.3% of five-year-olds in Enfield had received both doses of the MMR vaccine in 2024/25 - one of the lowest rates in the country.
A message currently posted on the NHS Ordnance Unity Centre For Health GP surgery's website said there is a "fast-spreading measles outbreak in several schools across Enfield".
It added: "Infections have been confirmed across at least seven schools in Enfield and Haringey and it is spreading.
"During this recent outbreak, one in five children have been hospitalised due to measles and all of them had not been fully immunised."
Enfield Council said it is "working closely with the UK Health Security Agency, the NHS and local partners to respond to a confirmed outbreak of measles in the borough".
Alev Cazimoglu, the council's cabinet member for health and social care, said: "The current outbreak has mainly affected children and some have required additional care with a short stay in hospital.
"Measles is one of the most infectious known diseases. It is approximately six times more infectious than Covid-19.
"Catching the measles is entirely preventable, but it spreads extremely quickly where vaccination levels are low. By checking your family's vaccination status and taking up the free NHS vaccine, you are protecting not only yourselves but also vulnerable members of our community.
"Vaccination is the most effective way to protect yourself and your family. We urge everyone who is not fully vaccinated to act now."
There is no treatment for measles, only the vaccination to prevent catching it, which is part of the measles, mumps, rubella, varicella (MMRV) injection.
Two doses of a measles-containing vaccine provide high levels of protection and help prevent further outbreaks.