The planned sentencing of six people for a firearms conspiracy could not go ahead on Friday because the dock was too hot.
It came as the UK recorded its hottest-ever June day, with Wattisham in Suffolk recording a temperature of 36.9C, amid a days-long heatwave.
A red heat health warning is in place for London until 9pm.
The defendants, who included one of Pc Sharon Beshenivsky’s killers Faizal Razzaq, were due to learn their fates at Harrow Crown Court, which has been newly refurbished.
But the hearing had to be postponed because the temperature could not be brought down enough for them to sit in the dock.
Security staff said they could not put a fan in the dock, and the hearing could not start until the temperature dropped below 26C.
The building, which reopened earlier this year after a £26 million safety overhaul, has air conditioning. However, this was not sufficient to cool the dock.
Judge Hannah Kinch said: “The temperature will need to be monitored and if it goes above 26C the defendants will have to be taken down again.
“I don’t see how we can have a hearing while we are stopping and starting.
“I also think it would be deeply unfair to have defendants who are facing serious sentences kept in difficult conditions in the dock.”
At midday, the judge confirmed the hearing could not go ahead as the dock had not cooled down. It was adjourned until October.
Heatwave causes widespread disruption
The scorching heatwave has impacted hundreds of schools, shops and offices in London, with many closing.
It is also causing disruption on the railways, with multiple train operators urging passengers not to travel.
A number of London tourist attractions were closed on Thursday amid the sweltering heat, including the Cutty Sark, the Young V&A and part of Hampton Court Palace.
Changing of the Guard ceremonies at Buckingham Palace and Windsor Castle have also been suspended for the rest of the week due to “excessive temperatures”.
Health chiefs are also warning of the impact the heatwave is having on services as they face significantly more life-threatening emergency calls.
Several hospitals have declared critical incidents, with University Hospital Southampton being forced to cancel a number of planned operations and some outpatient appointments.
