Numbers of measles cases in the UK have risen to their highest level in nearly 20 years, experts said.
Health experts are blaming low uptake of the MMR vaccine
Surrey and Sussex could have up to 156 cases and South Yorkshire may have 180. Last year there were just 77 cases across England and Wales.
The south east's health agency blames low uptake of the MMR vaccine. It comes as a doctor who linked MMR with autism faces possible misconduct charges.
A boy's death from the disease in April was the first UK fatality in 14 years.
The Surrey and Sussex Health Protection Agency (HPA) said there had been 106 confirmed cases in the two counties with 50 more unconfirmed.
In Doncaster alone there have been 100 reported cases, with as many as 80 further cases elsewhere in South Yorkshire.
Details of the outbreak were first revealed on the BBC's Radio 4 show Woman's Hour.
Dr Wakefield could face a misconduct hearing over MMR
Dr Peter English, consultant at the HPA, said: "We haven't had an outbreak as big as this in this country since the MMR jab was introduced in 1988."
He said the outbreak was not a surprise as low MMR uptake had left "a lot of people now susceptible to the disease".
Parents have been warned the highly-contagious disease could bring on breathing difficulties, diarrhoea, ear, lung and eye infections and even swelling of the brain.
Dr English, urging parents to ensure their children have the initial vaccine and the booster jab, said without fast action there could be an outbreak similar to that in Dublin in 2000, when two children died and hundreds were hospitalised.
Earlier this week, it was revealed that Dr Andrew Wakefield, the doctor who first suggested a link between the MMR vaccine and autism, could be charged with serious professional misconduct.
Shadow Health Secretary Andrew Lansley said: "The government's failure to maintain confidence in the MMR jab has led directly to these outbreaks of measles.
"Where there is a persistent failure to secure sufficient take up of MMR to provide population wide protection, we have made it clear the government should consider whether to investigate protocols for single jabs.
"It would be better however if everyone understood that the MMR jab is safe and accepted immunisation for their children."