An incinerator in Cardiff is pouring out black smoke during testing just days before it is due to be set to work burning 350,000 tons of waste a year.

Campaigners against the incinerator fear its twin smoke stacks will drop health-threatening airborne debris and particles across the city.

They are taking action in the High Court in the hope of stopping the facility from operating.

But they say even during testing, which began last week, the smoke stacks are belching out debris which is being dropped onto the city.

The incinerator will be operated by private contractor Viridor on behalf of a consortium of Welsh councils including Cardiff, Newport, Vale of Glamorgan and Monmouthshire.

Viridor is now taking in domestic, industrial and commercial waste from neighbouring local authorities and plans to start burning it next week, well ahead of the hand-over date scheduled for December 23.

Cardiff Against the Incinerator (CATI) is campaigning against its operation on health grounds arguing that Viridor’s test burns are already causing problems, with deposits from the smoke falling on the city.

“The incinerator is being tested now,” said William Prosser, a member of the CATI committee.

“We have been told by one professor that nano-particles which are emitted pass through the filters and can get into the lungs, into the blood, and into the brain because they are so fine. God knows what they will be burning.”

He said that two 90-metre high chimneys will spread smoke across the city, and that in Cardiff Bay there is a residential community which is itself 70 metres high.