Mr Johnson insisted that measures were in place to deal with the spread of the virus but urged people to adhere to good hygene practices to limit the risk to themselves and others.

He said: "We have been preparing for this for a long time. It came actually above terrorism as a threat to this country, so we had the whole Cobra machinery and inter-agency working. We have gone through simulation exercises where everyone was involved.

Mr Johnson told BBC1's Andrew Marr Show: "What we are finding is that it is attacking the young rather than the elderly, who traditional seasonal flu would attack.

"But in the vast majority of cases it is a mild illness. We have got stocks of anti-virals to cover over 50% of the population and a vaccine on the way."

His words come as mothers-to-be were advised to steer clear of crowded areas.

So far, more than 650 people have been taken to hospital with the virus in England, including more than 200 children.

Under-fives have been three times more likely than older patients to be hospitalised. In Scotland, 44 people have been taken to hospital, while 11 have been treated in Welsh hospitals. At least 146 cases have been recorded in Ireland.

Officially, there have been 29 British deaths involving swine flu, including four children and two mothers who died shortly after giving birth.

All except one – six-year-old Chloe Buckley, who died 10 days ago in west London — were known to have been suffering from underlying health problems.

Pregnant women are at increased risk of contracting any infection because their immunity is suppressed to ensure that their body does not reject their baby.

Experts believe that most cases of swine flu would not harm the mother or foetus, but rare cases could lead to premature labour or miscarriage or cause birth defects.

In Australia, where 11 expectant women with swine flu are in intensive care, pregnant women have been urged to stay at home when possible, and to wear face masks when they do go out.

Mask-wearing is not part of Britain’s strategy because it is thought to do little to reduce the spread of disease and encourages complacency against more useful measures, such as regular hand-washing.

Separate guidance to pregnant GPs and community nurses suggests that they should not see patients with suspected swine flu.

The Government has decided against advising women to delay pregnancy until the swine flu pandemic has passed, even though such a recommendation was contained in previous plans.

The number of people off work with symptoms of flu more than doubled last week, according to figures compiled for The Sunday Telegraph. On Friday , about 177,000 people were absent with flu-like symptoms, compared with 80,000 the week before.

Aaron Ross, the chief executive of FirstCare, an absence management company that carried out the survey of 1,000 companies, said businesses should offer home working to staff who are most vulnerable to swine flu.

telegraph.co.uk