A strain of flu that has caused widespread illness in Australia has arrived in Britain, according to the Health Protection Agency.
The Fujian-like strain has already claimed the lives of at least four of the five children who have died from flu so far.
While children die from flu every year, these deaths are earlier than usual, indicating that a larger outbreak is possible.
The strain was recently responsible for Australia's biggest flu outbreak for five years.
The HPA said it was too early to predict how it would affect Britain but urged the over 65s and "at risk" groups to get vaccinated against flu.
Britain has seen unusually low rates of flu for each of the past three winters.
The latest figures from the HPA suggest flu cases are rising to what is considered normal seasonal levels.
In recent years, most cases of flu in Britain have been caused by the Panama-like virus. The current flu vaccine is designed to protect against this strain.
However, the HPA said the main strain circulating this year is the Fujian-like strain and people are becoming infected earlier than usual.
"For the last three years, we have seen very low levels of flu in the UK but flu activity has started earlier than normal this year so we are expecting to see more cases," said Dr John Watson, a respiratory expert at the HPA.
"Laboratory testing so far this year shows that the main strain circulating is a flu A Fujian-like strain."
Dr Watson said children and adolescents have been most affected so far. There have been two deaths in England and three in Scotland.
"There have unfortunately been a few deaths due to flu reported in children in England over recent months.
"HPA laboratory testing has show that two of three cases resulted from infection with the Fujian-like strain."
The Scottish Centre for Infection & Environmental Health said tests had also shown that the strain was responsible for its three deaths.
"Unfortunately there have been three deaths in Scotland over the last two months in children between the ages of two and eight years," said Dr Jim McMenamin, a consultant epidemiologist at SCIEH.
"UK Influenza Laboratory tests have confirmed the presence of the Fujian strain of influenza."
These deaths are unusual in that they are earlier than normal. However, over winter around three children die from respiratory illness in England each week.
Dr Watson urged the over 65s to get vaccinated against flu. He also suggested children over six months who fall into "at risk" groups should also receive the jab.
"The current flu vaccine contains the Panama-like virus and is considered to offer some protection against the Fujian-like strain and good protection against other strains that may also circulate," he said.
"When flu strains change over time, they are more likely to affect the young who will have less immunity to them.
"For this reason, it is especially important for children over six months of age who fall into the at-risk groups to be vaccinated."
Those in at risk groups include those with severe asthma, people with chronic kidney disease, diabetes or with lowered immunity due to any disease or treatment such as steroid medication or cancer treatment.
The Department of Health has written to doctors to make them aware of the situation and remind them of the guidance on vaccinations and treatment.
The strain has already caused havoc in Australia and to a lesser extent New Zealand this year.
Hospitals in many parts of Australia were forced to cancel non-urgent operations because they needed beds to treat people with flu.
The strain spread fast and some schools in many cities reported that one in three pupils were off sick at the peak of the outbreak.