At least 15 children have now died of Strep A, official figures show.
Strep A, a common and highly infectious bacterial infection, causes Strep throat or tonsillitis, as well as scarlet fever, impetigo and, in rare and severe cases, iGAS.
Official data from the UK Health Security Agency show that there have been 13 deaths in England in under-18s since the start of September, up from five last week.
One child in Wales and one in Northern Ireland are also known to have died of the bacterial infection. There have been no reported deaths in Scotland.
Five children under the age of five have died of iGAS in England, five between five and 10 years old and three between 10 and 14 years old.
In the winter of 2017-18, the last time there was a high level of Strep A in circulation, 27 children under the age of 18 died of iGAS in the entire season.
There have been 85 iGAS cases in children aged one to four, the data show, and 60 cases in children aged five to nine.
The UKHSA said 60 people of all ages have died of invasive Group A Strep (iGAS) in the last few weeks as cases soar, and 23 of the deaths were in people over the age of 75.
Health officials said the rate of iGAS cases in the elderly is similar to what it was before the pandemic.
Figures suggest iGAS is just as deadly as it has been in previous years but that more children are catching it as a result of not being exposed to the virus in recent years.
Since Sep 12, there have been 6,601 cases of scarlet fever – more than twice as high as the same time period in 2017-2018.
Dr Colin Brown, the deputy director of the UKHSA, said: “Scarlet fever and Strep throat are common childhood illnesses that can be treated easily with antibiotics.
“Please visit NHS.uk, contact 111 online or your GP surgery if your child has symptoms of this infection so they can be assessed for treatment.
“Very rarely, the bacteria can get into the bloodstream and cause more serious illness called invasive Group A Strep. We know that this is concerning for parents, but I want to stress that, while we are seeing an increase in cases in children, this remains very uncommon.
“There are lots of winter bugs circulating that can make your child feel unwell, which mostly aren’t cause for alarm. However, make sure you talk to a health professional if your child is getting worse after a bout of scarlet fever, a sore throat or respiratory infection.
“Look out for signs such as a fever that won’t go down, dehydration, extreme tiredness and difficulty breathing.”
Officials urged parents to ensure that children are washing their hands with soap for 20 seconds, catching coughs and sneezes in tissues and isolating them when unwell.