The Old Bailey will remain closed for the rest of the week after an “accidental” fire broke out inside the high-security building.
On Wednesday morning, 1,500 people were evacuated and two had to be rescued from lifts after a fire broke out inside the Central Criminal Court in central London.
The building, which houses 18 courtrooms, will now remain closed for emergency works for the rest of the week, it was announced.
On Thursday afternoon, the Recorder of London, Judge Mark Lucraft KC, said there was still no power in the building due to the fire in an electricity substation.
UK Power Networks was working on the substation and new kit needed to be installed, he said.
It would not be in place in time for a “safe operation” on Friday and it was hoped the Old Bailey would reopen on Monday, he said.
The Old Bailey is the world’s most famous criminal court and has high levels of security as it hears major criminal cases, including murder and terrorism trials.
In the past, it has been identified as a high-value terror target and was badly damaged in an IRA car bomb attack in 1973.
A spokesman for the London Fire Brigade said the incident was being treated as “accidental” due to an electrical fault.
However, a terrorism expert told the PA news agency of a recent trend among far-right extremists to advocate “infrastructural attacks” on nuclear, electricity and water supplies.
Professor Matthew Feldman said: “Some neo-Nazi revolutionaries have turned towards infrastructural attacks as potentially less guarded and secure and something that could set off wider societal breakdown after which they can take control.
“A year ago, neo-Nazi Brandon Russell, who has links with far-right extremists in the UK, was charged with the Baltimore power grid plot in the United States while he was on parole for explosive offences.
“Whether it is just a coincidence or something that does have some causality about it, there is no doubt this is something that is being increasingly advocated by neo-Nazi revolutionaries.”
On Wednesday morning, a member of security staff had raised the alarm after hearing a loud explosion, the London Fire Brigade said.
The lights went out, plunging some windowless courtrooms into complete darkness, as acrid black smoke was seen billowing from the rear of the building in Warwick Lane.
Four fire engines and 25 firefighters were called to the scene as crowds of barristers in black gowns and wigs gathered at the front of the building.
Later, defendants were seen being escorted by police out of a side exit and into waiting prison vans to be returned to jail.
A London Fire Brigade spokesman confirmed the blaze started within the Old Bailey building and said the likely cause was “accidental”.
He told PA: “We can confirm that the fire was within the Old Bailey office block.
“There was a security officer that initially described hearing an explosion from an electricity substation on the ground floor and that was the first of seven emergency calls to London Fire Brigade.
“Four fire engines were involved in tackling the fire, with 1,500 people evacuated.”
He said firefighters had to wait for UK Power Networks before they could safely enter the area to tackle the blaze.
He added: “Following an assessment, the fire was found to have burnt itself out within the substation.”
“An electrical event has taken place associated with the substation. There was nothing suspicious about the fire,” he said.
He added: “Because the electrical supply went down, we led two people stuck in lifts to safety. We did assist two people. They were absolutely fine.
“It looks like damage was contained to just the substation.”
Cases which had been due to be heard at the Central Criminal Court on Thursday were put off.
They included the ongoing trial of Constance Marten and her partner, Mark Gordon, for the manslaughter of their baby daughter, and the televised sentencing of three people for the “sadistic” murder of Shakira Spencer.
Urgent bail hearings were moved to Southwark Crown Court, a Ministry of Justice spokesman said.
Other urgent hearings such as custody time extensions will be moved to other courts.
A spokesman for UK Power Networks told PA that it would be investigating the electrical fault in its substation.
In a statement, the company said: “UK Power Networks engineers were called to Paternoster Square in London at 11.10am yesterday after a fire was reported which has affected power supplies.
“We are working in conjunction with the customer, whose preference was for a permanent repair rather than a temporary fix.
“Our teams are working safely, round the clock to replace electrical equipment which was substantially damaged by the fire, which will take several days to replace.”
On Wednesday, temporary chief inspector Tom Fisher, of City of London Police, said: “There is no indication at this stage that this is anything other than an incident involving an electrical substation.”