The trial of Constance Marten and Mark Gordon had to be postponed for the day after the Old Bailey was evacuated because of a fire.

Courtrooms were plunged into darkness this morning before an announcement told those inside a fire had been reported.

A witness said that a "series of explosions" could be heard from a building next to the Old Bailey, with smoke seen pouring from the building.

The air smelt of soldering metal.

Some 25 firefighters descended on Warwick Lane and a cordon was put up around the area, including tape that warned people not to cross the “hazard zone”.

It is believed the incident involved an electrical substation near the rear of the court.

UK Power Network engineers attended to try to fix electrical supplies.

More than a thousand people, including barristers in wigs and gowns, jurors and press were evacuated from the building.

Crowds waited outside to see if they would be allowed to re-enter the court but in the afternoon it was confirmed that would not be the case.

Read more: Recap: Constance Marten and Mark Gordon trial continues

In a video posted on social media, James Rossiter, director of communications for the Criminal Bar Association of England and Wales, said that all 17 courtrooms had concluded business for the day.

He said: "Justice is delayed, but it won't be denied."

The court will reopen following a safety inspection. 

A spokesman from London Fire Brigade said: "Four fire engines and around 25 firefighters responded to an incident involving an electrical substation on Warwick Lane, near the rear of the Old Bailey.

"Firefighters worked with UK Power Networks to make the scene safe and bring the fire under control. The ground floor substation was partly damaged in the incident. Around 1,500 people were evacuated from the Old Bailey and surrounding area and there are no reports of any injuries.

"The brigade received the first of six calls reporting the incident at 10.44am and crews had the fire under control by 12.07pm. 

"The cause of the fire is under investigation."

City of London Police said there is "no indication" the fire was "anything other than an incident involving an electrical substation".

Chief Inspector Tom Fisher of 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 currently in attendance.

"There is no indication at this stage that this is anything other than an incident involving an electrical substation.

"London Fire Brigade reported that around 1,500 people were evacuated from the Old Bailey and surrounding area."

Defendants could be seen being picked up by Serco vans and taken away from the court. These included Mark Gordon.

Gordon, 49, and Marten, 36, are accused of gross negligence manslaughter, causing the death of a child, child cruelty, concealing the birth of a child and perverting the course of justice.

The couple’s baby Victoria was found dead in Brighton on March 1 last year.

They deny all the charges.