Measles is likely to spread rapidly across more parts of the UK unless more people take up the vaccine, a senior health official has warned.
Dame Jenny Harries, chief executive of the UK Health Security Agency, says vaccination rates are "well below" what is recommended by the World Health Organization.
Pop-up clinics are being introduced to get more children vaccinated as cases continue to rise.
Measles is a highly contagious disease.
It is spread by coughs and sneezes. More than 200 cases have been confirmed in the West Midlands in recent months, mostly in Birmingham.
The UKHSA said Dame Jenny has expressed concern that, without urgent action, we are likely to see the measles virus "seeding and spreading rapidly" in other cities and towns with low vaccine uptake.
She told OceanNewsUK: "The focus this morning is on the West Midlands, but I think the real issue is we need a call to action right across the country."
The UKHSA has now declared the measles outbreak a national incident, allowing it to put more resources into tackling the problem.
In some areas of London, nearly half of children have not been vaccinated against it.
The vaccine, the Measles, Mumps, Rubella (MMR) jab, is delivered in two doses, the first given at 12 months, and the second at about three years and four months, before children start school.
Dame Jenny said the UK had previously established an elimination status for measles, but vaccination rates have now dropped.
"On average about only 85% of children are arriving at school having had the two MMR doses," she said.
The standard MMR vaccine contains ingredients derived from pork, however those who do not eat pork products can request an alternative version called Priorix from their GP.
WHO recommends two-dose measles vaccination coverage of at least 95% of the population.
In London, mobile vaccination clinics have been set up to increase take-up of the jab in the borough of Camden, where one in four children start school behind on their measles vaccination.
Kirsten Watters, Camden Council's director of health and wellbeing, said the clinics offered a convenient place for busy parents to tick the jab off their list.
"When talking to parents, we find most do intend to vaccinate their child, we've got high levels of confidence and trust, it's just that they're finding it difficult to organise appointments and get to those vaccination clinics," she told the Today programme.
People have forgotten how miserable it is to contract measles, Dame Jenny said.
"I'm actually the generation that had measles, and I cant remember much from my childhood, but I can remember it and it is absolutely miserable," she said.
What happens if you catch measles?
Measles is recognised by a high fever, a blotchy red or brown rash, sore, red and watery eyes, coughing and sneezing.
It normally clears up after seven to 10 days, however, it can lead to serious problems if it infects other parts of the body, such as the lungs or brain.
Complications can include pneumonia, meningitis, blindness, and seizures.
Babies and young children, pregnant women and those with a weakened immune system are more at risk.
People with measles are infectious until at least four days after the rash appears.
Measles is one of the most infectious diseases in humans. On average, in communities with low protection, one person will spread the virus to 15 others.
That makes it far more infectious than coronavirus, which has an R or reproduction number of about 3.
The R is a way of rating a disease's ability to spread.
Those with mild symptoms are asked not to visit their GP or hospital but to call the NHS on 111 or get help online.
They should also stay away from nursery, school, university, work and other group activities while they are infectious.
