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Emergency workers are simulating a terrorist attack on London's subway system, a pre-Olympics security exercise that's scraping up unpleasant memories for some Britons.

The two-day test mimics an attack on Aldwych subway station, an abandoned transit hub in Westminster that's been closed to commuters for nearly 20 years.

Known as "Exercise Forward Defensive," the test is meant to prep British police, fire and ambulance staff for the worst ahead of the 2012 London Olympics.

"They're really trying to figure out how to respond to an event in this country that would take place at the height of the Olympics," said Â鶹ӰÊÓ' London correspondent Ben O'Hara-Byrne.

Authorities have refused to disclose what kind of scenario crews are rehearsing but for some the idea of a mock terrorist attack on London's bustling transit system is enough to evoke memories of a real-life attack on July 7, 2005.

In the so-called 7/7 attacks, four suicide bombers killed 52 commuters on three subway trains and a bus. The assault occurred one day after London was told they'd be hosting the 2012 Olympics.

O'Hara-Byrne told CTV's Canada AM that Wednesday's security exercise is supposed to be reminiscent of the 2005 attack.

"They'll be trying to learn from mistakes that were made that day, some of the things that went wrong, trying to make sure they're improved," he said from London.

All levels of emergency services, from Britain's top-ranking Cobra emergency response and co-ordination committee to individual constables, will be participating in the exercise.

"This is focusing on not only the evacuation, the rescue effort, the crime scene securing but also how authorities communicate with each other," said O'Hara-Byrne.

He added that personnel are focusing on honing communication skills in particular because a lack of communication was highlighted as a problem in the response to the 7/7 bombings in 2005.

Valentina Soria, a terror threat analyst with the U.K.'s Royal United Security Institute, said emergency personnel started analyzing Olympic security in September.

"I think they've come a long way to understand where improvements need to be made," she told Â鶹ӰÊÓ Channel on Wednesday.

She said security has been bolstered as a result of previous exercises but terrorist threats, particularly from "lone wolves" rather than groups, are always a concern -- especially at an event broadcast live around the world.

British Transport Police spokesman Simon Lubin told The Associated Press that the exercises are meant to instill confidence in emergency personnel.

"If there are mistakes, this is the time to make them, not when there's a real incident," he said.

British authorities have doubled London's security budget to $1.5 million ahead of the games. About 13,500 military personnel and 12,000 police officers will be on duty during the games between July 27 and Aug. 12.

Despite the elevated police presence, O'Hara-Byrne said officials are trying to keep their efforts discreet to avoid intimidating revellers.

"There's sort of a two-pronged attempt here to not only prepare for the worst but to also make sure that security isn't invasive and isn't omnipresent."