A mistake by an electrical engineer may have sparked the which caused to close on Friday, a source said.

Bosses at the hub - Europe's busiest airport - as a result of the fiasco. The led to and power cuts across west London.

More than 1,350 flights were either delayed or cancelled, which left tourists stuck abroad and unable to leave the UK. Some 120 transatlantic services were forced to turn round in mid-journey, it is understood.

, while Westminster sources have blamed human error. Those "familiar with the investigations" were pointing towards a mistake by an electrical engineer sparking the fire, reports website this morning. A source told the publication: "It's always cock-up rather than conspiracy."

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Last night, but bosses faced pressure over the debacle. Willie Walsh, director-general of global airlines body IATA and a former head of , said: "Firstly, how is it that critical infrastructure – of national and global importance – is totally dependent on a single power source without an alternative. If that is the case – as it seems – then it is a clear planning failure by the airport."

The fire at the substation ravaged the transformer - a device which transfers electrical energy from one circuit to another or multiple. It also damaged Heathrow's backup transformer and so the hub could not receive any electricity for most of Friday. The biomass power plant and diesel backup generators at Heathrow itself can only power essential safety systems, such as lighting and exit doors.

, by which time severe disruption had already happened. However, Yard later said it was "not treating this incident as suspicious, although enquiries do remain ongoing."

But Phil Hewitt, director of energy analysis firm Montel Group, said: "This potential lack of resilience at a critical national and international infrastructure site is worrying. An airport as large and as important as Heathrow should not be vulnerable to a single point of failure."

The airport hopes full operations will resume on Saturday. Heathrow chief executive Thomas Woldbye said the airport had faced a situation "as bad as it gets for us".

Supply had to be "restructured" to draw on the other two substations, which involved restarting thousands of different systems – which takes "an enormous amount of time", he said.

"We are very sorry about all the inconvenience. We lost a major part of our power supply. This was an incident of major severity. We lost power equal to that of a mid-sized city.

"Our backup systems have been working as they should, but they are not sized to run the entire airport. This is unprecedented. Everyone has been shocked but all our procedures have been working as they should."