An outbreak of measles at two schools in Hounslow could spread to other areas in west London, Public Health England (PHE) has warned.
The shocking alert comes after a number of children tested positive for the highly infectious viral illness at Feltham Community College, in Feltham, and Oriel Academy, in Hanworth, as well as other younger children in the community.
The virus, which can lead to serious complications and is easily spread in unimmunised children and adults, can be contagious from four days before until four days after a rash develops.
Pregnant women, babies under six-months-old and people with weak immune systems due to HIV or receiving treatment for cancer are at increased risk.
Dr Deborah Turbitt, deputy director for health protection for PHE London, said: “This outbreak of measles is affecting children and teenagers who are unvaccinated or only partially vaccinated with MMR.
"We could potentially see further measles cases in Hounslow and wider areas of west London as a result of this outbreak so we are asking parents to ensure their children are fully vaccinated with two doses of MMR.
“We are working with the schools involved and the London Borough of Hounslow and NHS England to urgently inform parents of the situation."
Letters have been sent out to parents and carers of all children at Hounslow schools, early years centres and children's centres with information on the symptoms and to ensure children are fully immunised.
Two doses of measles, mumps, rubella (MMR) vaccine gives full protection against the three illnesses.
Local GP surgeries have also been alerted to the outbreak.
In the UK, the first dose of MMR is given at 12 months of age, and the second dose from 3 years 4 months. There is no upper age limit for receiving MMR.
Headteacher at Feltham Community College, Victoria Eadie, said: “At FCC we have been working very proactively with Public Health England and our school nursing team in response to the reported cases of measles.
"A letter was sent to all of our parents to inform them that we had cases of measles with information and guidance.
"A follow up letter asking for data update and offering immunisation was sent to parents of children for whom there was unclear measles vaccination history.
"Measles is avoidable and we have strongly advised our parents to ensure all their children are fully vaccinated.
"We held an MMR immunisation clinic for all of our students who had either not been vaccinated, or only had one vaccination.
"We are continuing to work with both Public Health England to ensure the risk of measles is minimised.”
Dr Khadidja Bichbiche, consultant in public health for Hounslow council, said: "It is never too late for children and young people to get vaccinated against measles.
"Parents of unvaccinated children, teenagers and young adults who have not received two doses of MMR should arrange for them to be vaccinated by their GP.
"If you are unsure whether your child has had two doses of the vaccine, please speak to your GP who will have a record.”