The UK's top environmental watchdog is to investigate whether bodies regulating the discharge of raw sewage into Belfast Lough are acting within the law.
The Office of Environmental Protection (OEP) was set up to monitor environmental compliance after Brexit.
It has announced that it is investigating the role of regulators in holding Northern Ireland's water company to account through enforcement.
It will look at discharges of raw sewage from what are known as combined sewer overflows or CSOs.
NI Water supplies both water and sewerage services in Northern Ireland and is a government funded company.
The investigation will determine whether the authorities have failed to comply with environmental law in regulating how untreated wastewater, including sewage, is discharged to Belfast Lough and its surrounding rivers.
A particular issue is the number of CSOs. They are household drains that take a combination of sewage and rainwater.
During heavy rainfall they are designed to discharge straight to the lough to prevent the system backing up and flooding homes.
There are almost 2,500 of them across the whole of Northern Ireland.
In the Greater Belfast area many are in breach of environmental standards.
Northern Ireland has a higher number of CSOs than other parts of the UK because it was traditionally cheaper to install them rather than a system which separated rainwater and foul water.
Northern Ireland Water said they are necessary to protect homes, would be prohibitively expensive to replace and that as they only operate at times of heavy rain, any sewage is greatly diluted by high volumes of rainwater.
It estimates that of the 18 million cubic metres discharged from the entire CSO network a year around 1-2% is raw sewage.
The authorities under investigation by the OEP are Stormont's Department of Infrastructure (DfI), the Department of Agriculture, Environment and Rural Affairs (DAERA) and the Utilities Regulator.
"The quality of our water is one of the most pressing environmental challenges we face," said Chief Executive of the OEP Natalie Prosser.
"Nature is under unsustainable pressure in Northern Ireland, and one of the biggest issues is how wastewater is managed.
"We'll be looking at whether the Utility Regulator and DfI have met their duties to take necessary enforcement action, where Northern Ireland Water may not be fulfilling their own responsibilities for sewage discharges.
"For DAERA, we’ll examine whether it has set and updated the conditions that allow sewage overflows to spill in line with legal standards," she said.
If the OEP finds non-compliance it will work with bodies to improve performance, though it can also take court action.
Northern Ireland Water said that while it was not the focus of the investigation it would co-operate fully with it.
"NI Water has consistently highlighted the urgent need for sustained investment in wastewater infrastructure to address long-standing challenges across Northern Ireland," it said in a statement.
It also pointed out that a programme which aimed to address the problems had been paused due to funding constraints.
The Department for Infrastructure, which is NI Water's funding department, also said it would co-operate with the inquiry.
Minister Liz Kimmins was not available for interview but her department said she would continue to seek greater investment for NI Water and explore the possibility of developer contributions.
The suggested imposition of domestic water charges to pay for investment in NI Water has been ruled out by politicians.
This will be the fourth OEP investigation in Northern Ireland.
