The London bombing highlighted important gaps in business continuity plans, according to security experts.
Many firms discovered, to their cost, that their business continuity plans relied on being able to communicate with key staff via mobile phone networks, which were out of action or unreliable for most of the day the bombs exploded.
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Others found themselves in difficulty when key staff were unable to make it into work, said Andy Tomkinson, a director at the Business Continuity Institute.
"Companies did panic. They did not know how to look after their staff. And communications plans did not work because mobile phones were congested," Tomkinson said.
In the aftermath of the explosions police invoked a system called Access Overload Control, which shuts down large swathes of the mobile network, to free-up communications for the emergency services.
"Reliance on mobile phones is a big question-mark. It is in people's business continuity plans, but mobile phones let them down," he said.
Corporate e-mail systems also came under strain, which in some cases caused severe disruption to businesses.
Some companies, such as Sainsbury's, instructed staff to send text messages rather than make mobile phone calls - a lesson learned from the central London power cut two years ago.
"It was extremely difficult to get a line. We understood that text messaging is far more effective at getting messages through," said Steve Mellish, head of business continuity.
Analyst firm Gartner said that the attacks showed that organisations need to have viable, tested business continuity plans, which are focused on people, not just business assets.