Two families had to be rushed to hospital after builders let a boiler pump carbon monoxide back into a flat, a court heard.

Workers on the council block in the Abbey Estate, off Abbey Road, St John’s Wood, covered over a chimney while renewing the felt covering on a flat roof, it was said.

Residents believed they were coming down with flu as they succumbed to the deadly gas. Within hours, two had collapsed and others were experiencing “severe symptoms” of carbon monoxide poisoning, the Old Bailey heard on Monday.

Apollo Property Services allegedly failed to protect residents from the “material risk” of carbon monoxide poisoning during their work on the block on March 17, 2008.

Apollo was the principle contractor on a complete refurbishment of the estate, and the low-rise Exonbury block was the first to be updated.

Prosecutor Claire Harden said: “In simple terms, they covered over where a chimney had been.

“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 affected residents woke at 1am “with a severe headache, the likes of which she hadn’t had before”, said Ms Harden.

“She went downstairs to get some painkillers and collapsed on the kitchen floor. Her husband was in the bathroom being unwell. Their son came down and tried to help her get up but she simply couldn’t.

“Her heart started racing and she was sweating. She thought she was having another heart attack.”

A family of four in a neighbouring flat also succumbed, even though they were not connected to the covered chimney.

Apollo Property Services, based in Waltham Abbey, Essex, deny failing to discharge a duty to non-employees under the Health and Safety at Work Act. The trial continues.