Pressure groups said the facility is not monitored closely enough

The Environment Agency said incidents at the Eastcroft Incinerator happened between 20 December and 3 January - caused by a boiler tube failure.

The agency said at no time was public health or the environment in danger.

But people opposed to Eastcroft's expansion argued it is another reason for refusing planning permission.

Third furnace

Jon Beresford, from Nottingham Against Incineration and Landfill (NAIL), said: "Carbon Monoxide is the gas at the centre of this breach but there are hundreds of chemicals released by incinerators.

"Only six of these are constantly monitored so we have no idea what else has been released.

"This is further evidence of why the city council should say no to the proposed doubling of capacity in this incinerator."

The agency has approved a licence to operate a third incinerator at the site near Nottingham city centre.

The expansion at Waste Recycling Group's Eastcroft incinerator still needs planning approval.

The third furnace would burn an extra 100,000 tonnes of rubbish a year on the Eastcroft site.