Security experts feared that last summer's Olympic Games opening ceremony would be targeted by a "catastrophic" cyber-attack.
Oliver Hoare, head of cyber security for the Games, told the BBC that he received a phone call from intelligence agency GCHQ on the day of the ceremony which suggested it could be attacked.
Officials believed that the lights could be switched off during the multi-million-pound ceremony.
The threat never materialised, but Hoare added the team ran extensive test and precautionary measures in preparation.
He said: "There was a suggestion that there was a credible attack on the electricity infrastructure supporting the Games.
"And the first reaction to that is, 'goodness, you know, let's make a strong cup of coffee and get into the office'.
"We'd tested no less than five times the possibility of an attack, a cyber-attack, on the electricity infrastructure.
"In a sense I think we felt pretty well prepared, but there's always an amount of concern, particularly when you've only got eight or nine hours before the opening ceremony."
Hoare recalled that, when the team assessed the threat and put in a contingency plan ahead of the opening ceremony, he was so confident that even if "all the lights went out in east London you could guarantee that the Olympic Stadium would still be burning brightly".
"We effectively switched to manual, or had the facility to switch to manual", he added. "It's a very crude way of describing it, but effectively we had lots of technicians stationed at various points."
Twitches about switches
Despite this back-up, Hoare said he "twitched" every time the lights in the stadium dimmed as he watched the ceremony.
Just an hour before the ceremony began, the security chief was assured by a colleague that "if the lights go down we can get them up and running regardless within 30 seconds".
However this did not comfort Hoare as intended, given the ceremony was to be seen by millions around the world.
"Thirty seconds at the opening ceremony with the lights going down would have been catastrophic in terms of reputational hit," Hoare said. "So I watched the opening ceremony with a great deal of trepidation."
David Harley, senior research fellow for ESET: "The nature of the threat in these reports isn't entirely clear, or how much substance there was behind those fears of a direct attack on UK power utilities or the Olympic Games in general, but the business world in general might learn something about risk assessment and contingency planning from this report."