The number of cases of measles has almost doubled following two outbreaks of the illness, health officials said.
In England and Wales there were 964 cases in the first six months of this year - compared to 497 during the same period in 2011, the Health Protection Agency (HPA) said.
A spokesman said the cases have been associated with ongoing outbreaks in Merseyside and Sussex, and a small number of cases have occurred in England's travelling communities.
The number of cases of rubella - also known as German measles - have also increased.
The HPA said 57 cases of the viral infection were reported between January and June - exceeding the annual totals for the previous nine years.
Officials said parents should ensure that their children have received the MMR jab - the combined vaccine that protects against measles, mumps and rubella - before going back to school.
Dr Mary Ramsay, head of immunisation at the HPA, said: "It's extremely concerning that measles cases are continuing to rise.
"Measles can be very serious and parents should understand the risks associated with the infection, which in severe cases can result in death.
"It's vital that children receive both doses of the MMR vaccination and ahead of returning to school after the holidays, we are urging parents to ensure their children have received the two doses, which will provide the best protection against the risks associated with measles, mumps and rubella.
"If you are unsure if your child has had two doses of the vaccination, speak to your GP who will have a record of which vaccinations your child has received."
Adam Finn, Professor of Paediatrics at the University of Bristol, said: "Measles is an entirely preventable disease so it is very sad to see that the numbers of cases are going up.
"It's a thoroughly unpleasant illness and, from time to time, unpredictably, can turn very serious or even fatal.
"You can't get too many doses of vaccine, only too few - anything less than two - so if you are in doubt about yourself or your child, find out and, if necessary, get the MMR vaccine which will protect you against mumps and rubella as well. You can be immunised safely and effectively at any age."