Measles outbreaks at three London schools have seen headteachers across the north and west of the capital put on high alert by health authorities.

Public Health England (PHE) has written to educators in the boroughs of Hammersmith and Fulham, Kensington and Chelsea and the City of Westminster warning them to be vigilant for the highly contagious infection.

The agency would usually contact affected parents and headteachers directly but the risk of unvaccinated family members attending other schools triggered the wider alert, PHE said.

Measles has so far been identified in pupils at Fulham Boys School, Chelsea Academy and St Marylebone Church of England School. Any unvaccinated siblings may also be at risk.

In a letter sent to head teachers, PHE consultant in health protection Dr Janice Lo said “there has been a significant increase” in measles cases amongst north west London residents since April.

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“Some of these children may have siblings attending other schools in nearby boroughs, and therefore, we would like to inform you of the affected schools and ask you to stay vigilant for any new cases reported,” she added.

“As measles is a highly infectious disease and can lead to serious complications, we strongly encourage you to take this opportunity to ensure your staff and parents are aware of the importance of measles, mumps and rubella (MMR) vaccine, and encourage them and the children to be vaccinated.”

A spokesperson for Fulham Boys School told The Independent there had been 14 confirmed cases at the school, but parents had been notified immediately and vaccinations were offered to all pupils.

It comes after warnings that half a million UK children missed out on the childhood measles vaccine in the past eight years.

PHE warned last week that hundreds of cases of measles had already been diagnosed England in the first three months of 2019.

Outbreaks have so far been contained to groups with low vaccination rates, such as the orthodox Jewish community, or those with links to other countries with outbreaks, including several eastern European states.

But PHE said there were signs it was spreading into the wider population.

Measles in the UK reached a decade high last year, and health secretary Matt Hancock has refused to rule out considering compulsory school vaccination schemes in future.

Globally cases rose 300 per cent around the world, according to World Health Organisation data.

The WHO is one of several organisations to blame the rise, in part, on the coordinated efforts of “anti-vaxx” groups spreading damaging myths about the MMR jab’s harms on social media.

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Dr Yimmy Chow, consultant at PHE’s London health protection team, said: “Protecting your children with two doses of the MMR vaccine is the best way to ensure they are not at risk of catching measles, mumps or rubella and preventing their spread to others who cannot be immunised such as newborn babies, pregnant women and those with weakened immune systems who are most vulnerable to infection.