Counter-terrorism police are now leading the probe into the fire which led to the Heathrow Airport shutdown.
Detectives said there is “currently no indication of foul play”, but all lines of enquiry are being examined, it is understood.
The elite police officers are leading the investigation because of “the impact this incident has had on critical national infrastructure” and their “specialist resources and capabilities”.
More than 1,000 flights could be diverted, cancelled or forced to return to their point of origin, triggering chaos across the World.
A spokesperson for the Met said: “We are working with the London Fire Brigade to establish the cause of the fire which remains under investigation. While there is currently no indication of foul play we retain an open mind at this time.
“Given the location of the substation and the impact this incident has had on critical national infrastructure, the Met’s Counter Terrorism Command is now leading enquiries.
“This is due to the specialist resources and capabilities within that command that can assist in progressing this investigation at pace to minimise disruption and identify the cause.”
Some 100,000 homes in west London were also without power overnight as 70 firefighters and 10 fire engines battled the blaze on Nestles Avenue in Hayes.
London Fire Brigade said the fire is under control and London Ambulance Service confirmed that there were no casualties.
Fears immediately surfaced that the crisis had been triggered by a Russian sabotage attack. The Kremlin’s spies are known to be recruiting criminals to carry out arson attacks and targeting transport networks.
Asked if foul play could be ruled out, Energy Secretary Mr Miliband told LBC: “There’s no suggestion that there is foul play.” Pressed on whether it was simply a “catastrophic accident”, Mr Miliband said: “The conversation I’ve had is with the National Grid, the chief executive of the National Grid and certainly, that’s what he said to me.”
Commander Dominic Murphy, Head of the Met’s Counter Terrorism Command, warned a Bulgarian spy ring convicted of spying on an “almost industrial scale” is a “clear example” of Moscow “contracting” out its intelligence gathering to proxies and criminals, adding: “We will see more of that.”
Asked if there were other more spy cells operating in Britain, Commander Murphy said: “The reality is, it won’t be the only activity Russia has conducted here in the UK and we have seen other disruptions into other types of Russian activity.
“Ken McCallum has been very open about the fact that there has been a very regular stream of sabotage and other activities linked to the Russian intelligence services.
“In terms of specific groups, all I can say is that within that 20% demand, there are a number of other investigations linked to Russia but they are not necessarily of this type. We look at lots of international threats.”
MI5 Director General Ken McCallum said Russia and Iran are on a mission to cause “mayhem” with an increase in arson, murder, kidnap and sabotage conspiracies.
The threat Moscow poses on Britain’s streets is “getting worse”, he warned.
Mr McCallum said Russia’s GRU intelligence agency, which was responsible for the Novichok poisonings in Salisbury in 2018, is behind the “sustained mission”, adding that Putin’s “henchmen” are ramping up arson and sabotage operations across Europe “in the misguided hope of weakening Western resolve”.
Britain’s top intelligence officer said Russia and Iran are hiring criminals to carry out their “dirty work” and acting with “increasing recklessness”.
At least 24 alleged saboteurs were arrested last year after being recruited by Russian spy chiefs.
A wave of fires at arms factories and military-related industrial sites in the West has alarmed intelligence chiefs. Suspected plots have been detected in London, Germany, Czech Republic and Sweden.