The threat of swine flu must be taken seriously but it is not an "end of the world scenario", an influenza expert said.

As the World Health Organisation raised its pandemic alert level to five, Dr Alan McNally explained that this means the disease has been spread between humans in two countries – Spain and Mexico.

The senior lecturer and researcher into influenza diagnostics at Nottingham Trent University said no human-to-human cases have yet been reported in the UK, but some may occur.

He went on: "These are the world's experts, they don't get it wrong and they don't take decisions like this lightly.

"We've got enough Tamiflu to treat everyone so I don't think there's any need to worry, but there's more of a requirement for vigilance."

He said people buying their own supplies of anti-viral drug Tamiflu could have disastrous consequences.

Dr McNally said: "People buying their own Tamiflu is the worst thing they can do. It can create resistance in a community. People should leave the diagnostics and treatment to the experts, trust them."

He also warned that the time had come for people to take the swine flu threat more seriously.

"The vast majority of people have been going around treating this as a bit of a joke but that stage has now gone," the expert said. "The time has come to have a bit of common sense about it."

However, Dr McNally said that a global pandemic would not be "the end of the world".

He said: "People associate the word 'pandemic' with death and that's not the case. A pandemic simply means that we've got human-to-human transmission of a virus in several countries.

"Pandemic refers to a worldwide outbreak, it doesn't refer to multiple deaths. The two things don't go hand in hand.

"We will have a pandemic and there will be millions of flu cases between now and the end of the year but it doesn't mean it will be an end-of-the-world scenario."

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