A watchdog has said “serious failings” must be addressed after a man died from severe heatstroke after being kept in a transfer van and an unventilated court cell on one of the hottest days in London for years.

Rafal Sochacki, a 43-year-old Polish national, died at Westminster magistrates court on 21 June 2017.

An inquest jury sitting in central London concluded his death was most likely due to him being subjected to excessive heat.

Sochacki was arrested on an extradition warrant on 19 June 2017 and taken to Wood Green police station. A prisoner escort-services van operated by the outsourcing company Serco took him to the magistrates court on 21 June, a day when temperatures reached over 30C (86F) in central London.

The van taking Sochacki to court stopped at Charing Cross police station, where he spent 50 minutes in a cell inside the vehicle with the engine and air conditioning turned off.

Sochacki arrived at court at 10am and was taken to a cell, which was not ventilated. The air conditioning at the court had not worked for weeks and the portable air-conditioning units did not provide detainees with effective relief from the heat, according to Sue McAllister, the prisons and probation ombudsman.

The prisoner’s health deteriorated after lunch and he was found unresponsive in his cell at 2.45pm. He was found to have died of severe heatstroke and hypertensive heart disease. His body heat had reached at least 39.6C.

McAllister highlighted a number of serious failings in a report published on Tuesday. The report noted the prisons inspectorate found no organisation had a good overall picture of the situation or would accept overall accountability.

McAllister said: “I am very concerned that there were inadequate contingency plans when the court’s air conditioning failed.

“We found some apparent non-compliance by Serco staff in delivering their contracted service and we have drawn this to the attention of both Serco and those responsible for the management of their contracts at HM Prison and Probation Service and HM Courts and Tribunals Service.”

Deborah Coles, the director of the charity Inquest, said: “This shocking and preventable death must send alarm bells across the criminal justice system. That a vulnerable man in the care of the state can overheat to death is outrageous. Such blatant disdain for the most basic standards of health and safety was not only inhumane but proved fatal.

“There is an accountability gap in the inspection and monitoring of court cells and the treatment of detainees. This must be urgently addressed at a national level. Without this, there is the ever-present risk of future deaths and harms.”

Julia Rogers, Serco’s managing director for justice and immigration, said: “We have been working closely with the Ministry of Justice and already agreed new procedures to manage extreme temperatures in our vehicles and in the court custody suites. The MoJ will also be providing us with a new specialist heat sensory device, that was not previously required, to trial in the custody suites.”

An MoJ spokesperson said: “Lessons have been learnt from this tragic incident. We have established clear procedures when court cells reach set temperatures and when there are excessive delays in collections, and all of our buildings now have ready access to a defibrillator.”

• This article was amended on 3 July 2019 because the Serco-operated prisoner transfer van was mistakenly described as a police van.