Thirty thousand tonnes of waste are to be exported from Northern Ireland for incineration on the Spanish holiday island of Mallorca.
Northern Ireland Environment Agency has consented to the export of 15 shipments of waste to the Son Reus factory, which is being arranged by Portadown-based Natural World Products Ltd (NWP).
Opposition parties on the island have urged its regional government not to import waste from abroad and are now taking their fight to the central government in Madrid.
Green Party MLA Steven Agnew said Northern Ireland's Environment Minister Mark H Durkan owes the Assembly and the people of Northern Ireland an explanation.
"In the past we have buried our waste in a big hole in the ground, now we are simply setting fire to it.
"This isn't innovation, it's caveman thinking," he said.
"Exporting our waste makes a mockery of any supposed commitment to sustainable waste solutions.
"The Environment Minister has questions to answer on this issue," he said.
"Are we paying to export this waste? If so surely we would be better processing the waste in Northern Ireland, creating jobs here."
An NWP spokesman said the company exported a high-specification fuel engineered from the non-recyclable fraction of household waste streams that would otherwise end up in landfill.
"This renewable fuel is used as an alternative to fossil fuels in highly efficient Energy from Waste plants across Europe that generate power for local businesses and local communities," he said.
"Waste materials from which the fuel is engineered have already been put through a full mechanical biological treatment and recycling process."
He added: "And our innovative and sustainable approach helps local councils meet EU-derived landfill diversion targets and avoid heavy fines that would otherwise fall back on ratepayers."
The Mallorca government has argued that importing waste for incineration at the Son Reus factory will allow it to keep taxation low. It imports 60,000 tonnes from Catalonia. It has now made an agreement that 30,000 tonnes of waste will be imported from Northern Ireland for incineration but this will have to go on hold from June 15 until October due to tourist season.