A fire which broke out inside the Old Bailey and postponed the trial of Constance Marten and Mark Gordon is not being treated as suspicious despite the building’s status as a high-value terrorism target.

On Wednesday morning, 1,500 people were evacuated and two had to be rescued from lifts after a fire broke out shortly before the trial was set to resume.

The fire is believed to be accidental. The court is closed today and the trial of the couple has been postponed.

UK Power Networks said the “incident” in one of its electricity substations at the Old Bailey was “being investigated”.

The Old Bailey is one of the world’s most famous criminal courts and has high levels of security as it hears major criminal cases.

Currently Constance Marten and Mark Gordon are on trial following the death of their baby daughter Victoria while they were on the run.

The couple were arrested in Brighton and charged with gross negligence manslaughter, concealing the birth of a child, child cruelty, causing or allowing the death of a child and perverting the course of justice.

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 a crowds of barristers in black gowns and wigs gathered at the front of the building.

Later, defendants including Mark Gordon 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 said: “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.”

“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.

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.”