A measles outbreak in London has forced health officials to warn schools to be on high alert for fresh cases.
Public Health England said a "significant increase" in cases had been reported in north-west London in recent weeks, with children at three schools particularly affected.
The health agency has responded by writing to headteachers in the boroughs of Hammersmith and Fulham, Kensington and Chelsea and the City of Westminster to advise them to "stay vigilant" in an effort to contain the outbreak.
Cases are said to be confined to three schools - Fulham Boys School, Chelsea Academy and St Marylebone Church of England School - but, so concerned by the possibility of unvaccinated siblings at other schools passing on the highly infectious illness, PHE has issued a wider alert.
The fact that all the schools referenced in the letter are Church of England schools is coincidental and not evidence of lower vaccination rates among the Anglican community, a PHE spokesperson said.
PHE confirmed to ITV News that there had been 31 cases in north-west London in April and May so far, more than double the 14 cases recorded in the area between January and March.
The letter to schools comes amid heightened concern over growing outbreaks of the disease and vaccine hesitancy.
The head of NHS England, chief medical officer and health secretary have been among the high-profile figures warning of the severity of the issue in recent months.
Last year, cases across England more than tripled to 966 from 259 in 2017.
An estimated 219 cases have already been recorded in 2019, fuelled by outbreaks in London, the north-west and east of England.
Anti-vaccine rhetoric - revitalised by social media - has been cited as a factor. ITV News learnt in February that a parliamentary committee will investigate the issue and spread of misinformation on platforms such as Facebook and YouTube.
Health Secretary Matt Hancock has refused to rule out making child immunisation compulsory, accusing so-called anti-vaxxers of having "blood on their hands".
A Public Health England spokesperson said: "We strongly encourage parents across the country to ensure that they and their families have had two doses of the MMR vaccine."