A firm which hired bungling builders who left a boiler pumping deadly carbon monoxide into a flat during a botched renovation is facing an unlimited fine.

Contractors employed by Apollo Property Services covered over a chimney while replacing felt covering on the roof of a block of flats in the Abbey Estate off Abbey Road, St John’s Wood, the Old Bailey heard.

Two families had to be rushed to hospital following the blunder. They believed they were coming down with flu, but within hours two people had collapsed and others were experiencing severe symptoms of carbon monoxide poisoning.

The company, registered in Church Street, Waltham Abbey, Essex, denied failing to discharge a duty under the Health and Safety at Work Act to persons not in their employment, but were found guilty following a trial.

Judge John Bevan QC adjourned sentencing on Thursday until December 17 and asked to be provided with a breakdown of the company’s turnover, profits and directors’ salaries. Bosses may also be ordered to pay compensation to the two families.

Apollo was the principle contractor employed to refurbish the Camden Council-owned Exonbury low-rise block on March 17, 2008.

“In simple terms, they covered over where a chimney had been,” prosecutor Claire Harden told the court.

“It was live and it was used by a boiler present in one of the flats. When it was covered the boiler couldn’t work properly. There was nowhere for gases to escape, other than back into the flat.”

One of the residents woke at 1am with a severe headache, collapsing in the kitchen when she went to get some painkillers, the court heard. her husband and son were also affected, but managed to open a window allowing some of the fumes to escape.

Tony Leach, Apollo’s health and safety director, said: “We fully accept the decision of the jury and would like to take this opportunity to apologise unreservedly to the families affected by this incident.

“Safety is our number one priority and this was an isolated incident involving work carried out by a subcontractor operating outside of our approved methods.

“We have carried out a full review of all our procedures and training to ensure that all staff, and subcontractors, working on our schemes meet the high standards we expect.”