There are five levels of security arrangements in Britain and a "severe" threat level is only one down from the most extreme level which is "critical".

Although it is uncommon to have security arrangements set at "severe" for a long period, Johnson admitted that the London 2012 Games represent a major challenge, which is the reason why security will be on red alert come Games time.

He said: "It is obviously difficult to set security arrangements as ‘critical’ for a long period of time due to the resources involved so critical is the highest level of security we can provide for a long duration of time.

"The London 2012 Olympic are obviously a potential terrorist target and we are very aware of the bombing attacks on the city that took place in 2005 the day after we won the bid to host the Games so we are leaving nothing to chance.

"The level of security arrangement can go up or down based on the intelligence we have but it will not go below ‘severe’ during London 2012 Games due to the nature and profile of the event."

Every spectator and official is set to have their bag searched upon entry to Olympic venues and go through airport style security screening in what Johnson called "a necessary measure" at the World Press Briefing here.

Johnson added that there is likely to be a ban on spectators and officials from bringing liquids, including bottled water, into the Olympic venues.

There would be exceptions, with items such as baby food and small bottles of suntan lotion being allowed into the Olympic Park but Johnson is adamant the move is in the best interests of those attending the Games.

He said: "There will be a no-liquids policy.

"I think people will understand that it is for their safety."

Johnson did say that organisers are planning on making free water available inside all venues but said that London 2012 are still considering their policy regarding food and could yet prevent spectators bringing picnics to the Games.

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