Staff and students at a London school have been given anti-viral drugs
Nine new cases of swine flu have been confirmed - eight in England and one in Scotland - bringing the UK total to 48.
The English cases involve two adults and six children. One adult is from the East Midlands and the other cases are all in London.
One of the adult cases is associated with travel to Mexico, while the children attended the same school.
A 19-year-old man from Greenock is being treated in hospital in Glasgow after testing positive for the virus.
The Health Protection Agency (HPA) confirmed that the children were from a London school which has closed because several pupils already have the illness.
The agency said the children were swabbed earlier this week and "may well have acquired their infection before their school's closure".
It added that anti-viral drugs had already been given to all children and staff as a precautionary measure.
The HPA also said that the infected adult from the East Midlands is a close contact of a previously confirmed case.
The Scottish government said the man who has tested positive for swine flu is from Greenock.
He started showing symptoms after coming into contact with someone who had travelled to Mexico. His condition was described as "not serious."
Scottish Health Secretary Nicola Sturgeon said: "While this latest case in Glasgow is a second instance of onward transmission in Scotland, it is noteworthy that the contacts of his so far identified, who were showing flu-like symptoms, have tested negative - so as far as we know he has not passed it on".
The Department of Health said all the infections in the UK so far were mild, and had been diagnosed and treated early.
The department spokeswoman said the government was not complacent and would prepare for the possibility of a global pandemic.
"The UK's arrangements are continuing to ensure that we are well-placed to deal with this new infection," she said.
Ms Sturgeon said a containment strategy in Scotland had been successful in preventing "large-scale" spread.
Earlier, officials in Wales said five people were under investigation for swine flu, but a further 74 had been given the all-clear.