SMOKE billowed from windows at the back of the Old Bailey this morning after a "series of explosions" went on for 10 minutes.
Defendants were walked out of the criminal court and into a Serco van an hour after the fire began in central London shortly before 11am.
The was evacuated and power cuts have been reported in nearby buildings.
Trials were derailed after lights went off in the court rooms following a fire, suspected to be an electrical fault.
Warwick Lane smelt of heavy smoke as raced to the scene.
Lights began flickering at around 10.40am for about half a minute before they all went out in the building, leaving counsel, defendants, judges and press in darkness.
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A tannoy system then told everyone to immediately evacuate due to a fire alarm and outside security shouted for everyone to get away from the building.
By 12.30pm defendants including Mark Gordon were walked out of the Old Bailey and into a Serco van to be driven to Belmarsh.
He was trying to cover his face as he walked out.
Usually defendants are driven out of a tunnel instead of being walked out onto the street.
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What most of us heard was a loud bang and some black billowing smoke.
Judge Mark Lucraft
London Fire Brigade said in a statement: "Firefighters are currently responding to reports of smoke coming from a building on Warwick Lane."
Recorder of London Judge Mark Lucraft, who is presiding over Constance Marten and Mark Gordon's trial, told The Sun he believes the substation has gone.
He said: "What most of us heard was a loud bang and some black billowing smoke.
"Power went off in the building. As far as I'm aware it's the sub station situated in the Old Bailey building.
"We are waiting for UK power to come and see before the fire brigade can go in. They have contained it."
Judge Lucraft added it is unclear what is happening to the defendants at the moment but there will be arrangements in place.
A witness wrote on X: "Series of explosions heard from building next to Old Bailey for about 10 minutes before smoke emerges.
"Many buildings seem to have suffered power cuts."
A BBC journalist at the scene tweeted: "Black acrid smoke coming from rear of Central Criminal Court, Old Bailey - lights flickered inside before it was evacuated - firefighter says fire in section of building where electrical substation sits."
T/Chief Inspector Tom Fisher, City of London Police, said: "We were made aware of an incident at the Old Bailey at around 11am.
"Warwick Lane is currently closed and we are working with London Fire Brigade who are in attendance.
"There is no indication at this stage that this is anything other than an incident involving an electrical sub station.
"London Fire Brigade reported that around 1,500 people were evacuated from the Old Bailey and surrounding area."
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A UK Power Networks spokesperson said: "UK Power Networks engineers were called to Paternoster Square in London at 11.10am after a fire was reported which has affected power supplies.
"We will work to restore supplies as quickly as is safely possible."