The first British cases of deadly swine flu have been confirmed as fears of a global pandemic grow.

Two people tested positive for the virus and are being treated in isolation in hospital in Airdrie, near Glasgow, while seven more people among 22 who have been in contact with them have developed "mild symptoms" not confirmed as swine flu.

The pair developed symptoms after returning from a trip to Mexico, where almost 150 people have died following an outbreak of the H1N1 virus. The Foreign Office is advising against all but essential travel to Mexico.

The World Health Organisation (WHO) raised its pandemic alert level to four on a scale of one to six, indicating the infection can spread between humans and cause community-level outbreaks.

The disease has claimed 149 lives in Mexico with as many as 1,600 carrying the virus. There are also confirmed cases in Canada and Spain, and suspected cases in New Zealand, France, Israel, Australia and the Republic of Ireland.

Health Secretary Alan Johnson earlier said ministers have put in place "enhanced" port health checks on passengers arriving in the UK and will use its stockpile of anti-viral drugs if the virus begins to spread widely.

The Government said it has enough medicine to treat half the population.

The NHS has a stockpile of more than £500 million worth of the Tamiflu anti-viral drug, which has proved effective on patients in Mexico, and scientists are working on developing a vaccine against the new strain.

The European Union's health chief has warned non-essential travel to swine flu-hit parts of Mexico and the US be postponed.

The US Centers for Disease Control and Prevention said there are up to 41 cases of swine flu in five states, including 20 more cases at a New York school, while people who have had contact with confirmed cases are also developing flu-like symptoms.

World Health Organisation Director-General Margaret Chan said the outbreak, caused when the H1N1 strain associated with pigs crossed over to the human population, constituted a "public health emergency of international concern".

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