Hong Kong authorities are under increasing pressure to verify whether the freshwater supply at a public housing estate contains bitumen, a substance banned from use in pipes, with experts calling for an investigation to trace the source of mystery black particles and determine if contractors have violated any rules.

A source said the government would hold a press conference on Thursday afternoon to release more details of their test results.

The water issue at two public rental and subsidised estates at Queen’s Hill in Fanling has escalated after laboratory tests commissioned by at least two media outlets detected bitumen, a black, sticky substance obtained from crude oil, in the supply.

So far, the government has not disclosed the source, six days after residents complained about the black specks found in tap water.

“Bitumen is not allowed to be used as part of the inner coating of water pipes,” Wong King, executive president of the International Institute of Utility Specialists, said during a radio programme.

He also said that the incident was not normal, urging authorities to trace the origin of the bitumen and whether any parties had violated any rules.