The rapid rise in measles means children face being forced to self-isolate for 21 days if they have not been vaccinated against the disease.
Councils issued warnings to parents after it emerged that 160,000 cases could occur in the capital alone.
Health officials say low uptake of MMR (measles, mumps, and rubella) jabs has increased the risk of the potentially deadly infection.
Parents in London and surrounding counties were told that if a pupil is infected with measles, classmates and siblings who have not had both doses of MMR could be sent home and asked to self-isolate for three weeks.
It is another blow for children who missed months of teaching during the pandemic.
Councils issued warnings to parents after it emerged that 160,000 cases could occur in the capital alone
Official data for October to December 2022 shows the areas most vulnerable to a potential measles outbreak with over a quarter of children in London missing out on the MMR jab
NHS England data released earlier this year shows that MMR vaccine uptake plunged to just 88.6 per cent for one dose in two year olds, and to 85.5 per cent for both jabs among five year olds
London’s Barnet Council, was one of those sending letters to parents warning that their child could face a three week isolation period from school if they are identified a close contact of a measles patient.
‘Currently we are seeing an increase in measles cases circulating in neighbouring London boroughs, so now is a good time to check that your child’s MMR vaccination – which not only protects your child against measles but also mumps and rubella – is up to date,’ it read.
‘Any child identified as a close contact of a measles case without satisfactory vaccination status may be asked to self-isolate for up to 21 days.
‘Children who are vaccinated do not need to be excluded from school or childcare.’
The Telegraph reported a similar letter was sent by neighbouring Haringey Council.
The 21 day isolation period is based of guidance published in 2019 by the UK Health Security Agency’s (UKHSA) predecessor Public Health England.
Medics have been increasingly concerned that measles, a disease long kept at bay because of vaccines, could make a return due to declining uptake.
Meanwhile it has emerged that a total of 42 cases of the super-mutated Pirola Covid variant have been spotted in the UK, up from 36 last week.