An adult from Birmingham is among the first confirmed cases of swine flu in the UK.
Gordon Brown revealed the information during Prime Minister Questions (PMQs) at the House of Commons.
He also revealed that a 12-year-old girl from Torbay and another adult from London have also been diagnosed with the deadly disease.
Last night one Walsall family were facing an anxious wait for test results after a daughter showed possible symptoms. The family of four, who did not want to be identified, were tested at Walsall Manor Hospital after returning from a two-week trip to Mexico.
The daughter developed symptoms on Thursday. The family were told to stay at home in isolation for two days and are due to get the results today. Walsall’s director of public health, Dr Sam Ramaiah, said the town was now preparing itself in case of an epidemic.
All three new cases recently travelled to Mexico, the Prime Minister said. They displayed mild symptoms and were responding well to treatment.
The girl's school, Paignton Community College, has been temporarily closed and all other pupils offered anti-viral treatment.
Mr Brown said the UK is taking "the preparations that are necessary and the precautions to prevent the incidence of this disease in this country".
Measures include enhanced airport checks, expanding anti-viral stocks from 35 million to 50 million, ordering extra face masks and printing an information leaflet for every family.
Health Secretary Alan Johnson is to make a statement on the situation to the Commons tonight.
The extra masks will be used by health professionals such as nurses and doctors working with infected patients in the event of a pandemic.
The Department of Health has said the masks are not intended for members of the public, for whom they are thought to be of little use.
The information leaflets on the virus will be delivered across the UK in the coming days and will contain information about the flu outbreak and preventative messages.
The first two confirmed patients, Iain and Dawn Askham, of Polmont, near Falkirk, had been on honeymoon in Mexico and were being treated in isolation at Monklands Hospital in Airdrie, Lanarkshire, today.
Their parents said the newlywed couple were "shocked" at the positive test results.
Other countries with confirmed cases of swine flu are Mexico, the US, Canada, New Zealand, Germany, Austria, Spain and Israel.
In the US, government officials said swine flu killed a 23-month-old child, the first death reported outside Mexico.
Staff at Paignton Community and Sports College referred all inquiries today to the local health authority, Torbay Care Trust.
A spokesman at Torbay Care Trust could not confirm which of the school's two sites had been closed but said a statement would be issued with further details.