A spokesman for the Olympic Delivery Authority (ODA) refused to comment on the incident but added: “Testing is standard practice in all major security operations.

“Such tests have a key role in developing our capability to ensure that London 2012 is safe and secure and that we are best prepared to detect potential threats before and during the Games.

“Members of the public with tickets should be reassured that such exercises are being staged to ensure their safety, our number one priority.”

The Metropolitan Police also said it would not comment on the security breach.

London 2012 has been put at severe terror threat level, the second highest classification.

The Olympics is a symbolic target and this year’s Games also marks the 40th anniversary of the 1972 Munich Olympics where the Black September terror group killed 11 Israelis.

Risks from protest groups or natural disasters are also part of the detailed safety planning.

Last month the Ministry of Defence confirmed that 13,500 military personnel, more than the 10,000 that were deployed to Afghanistan, will be part of the 23,700 security force for the Games.

The deployment across all military services includes 3,500, rising to a peak of 7,500, who will provide venue security.

Around 5,000 servicemen and women will support the police and other civil authorities, providing specialist capabilities such as bomb disposal and dog teams.

Another 1,000 will provide logistical support.

There will also be a 1,000-strong unarmed contingency force for deployment in the event of an “Olympics-related civil emergency”.

The overall 23,700-strong security force will include a mix of military, private security guards and at least 3,000 unpaid London 2012 volunteers who will be used at the start of the security process.