Bournemouth Crown Court heard that Harbour Reach in Poole contained 261 flats and 79 town houses, while Caversham Road in Reading contained 60 flats.

These were heated by gas boilers installed on internal walls, with flue gases, including poisonous carbon monoxide, vented to the open air by flues installed in building voids.

In such situations inspection hatches must be provided to allow the flues to be periodically checked for safety.

At Harbour Reach, HSE inspectors found faults including gas leaks, and flues and gas supply pipes that had not been properly fitted – establishing that the initial installation work was substandard, and that the final checking and commissioning of the work was inadequate and ineffective.

There were no inspection hatches originally installed in any of the ceilings of the flats,  meaning that the flues could not be subsequently checked for safety.

As a result, the gas supply to all 340 gas meters on the Harbour Reach site had to be disconnected until remedial action was taken.

The investigation also revealed evidence of defects in the installations in 309 (or over 90%) of the dwellings at Harbour Reach.

At Caversham Road in Reading there were defects in the gas installation affecting 40 properties.

As at Harbour Reach, inspection hatches were not initially installed to allow the flue system to be inspected, and defects were found with both gas supply and flue installations.

DSI Plumbing and Heating Ltd, of Fawley, near Southampton pleaded guilty to two safety breaches and was fined a total of £10,000 and ordered to pay a further £1,000 in costs.

Two self-employed people who worked for DSI were also sentenced.

Robert Percival, of Poole, and Andrew Church, of Gosport, both pleaded guilty to a single safety breach.

They were both given a two year conditional discharge and were each ordered to pay costs of £250.

After sentencing, HSE Head of Operations, Harvey Wild, said: “These serious failings involving gas safety only came to light after residents started to complain about ill-health.

“It is fortunate that in this case, no-one suffered long term ill-effects but the consequences of the faults, if left undetected, could have been fatal.

“Gas installers need to make sure flues and pipes can be properly checked and all installations are checked for safety as part of commissioning to ensure lives are not put at risk.”