Two fuel pipelines shut at Valero's Milford Haven refinery
The use of two fuel pipelines at the Valero oil refinery in Pembrokeshire has been suspended by Wales' environment regulator.
The enforcement notice was issued after an oil spill affected water, land and wildlife along the coast in December and the first week of January.
Natural Resources Wales said the pipelines at Milford Haven would be shut until the pipes were no longer a threat to the environment.
A clean-up continues on the coast.
Valero said its other pipelines and infrastructure remained unaffected and operational capacity remained unaltered.
Last week, Valero estimated that between 7,500 and 10,000 litres (1,650 to 2,200 gallons) of a "heavy" type of fuel oil - the sort generally used in ships - had leaked overnight and the jetty was closed.
NRW, along with Pembrokeshire council and Pembrokeshire Coast National Park Authority, are monitoring and advising Valero and its contractors on the clean-up.
Since an initial report of pollution last month, work has been focusing on minimising the impact of the oil on the area and wildlife.
NRW said the oil from the leak had been contained.
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Andrea Winterton, NRW operations manager, said: "Protecting Wales' waters is a huge part of our job and we have acted to make sure that the estuary and its wildlife is protected.
"A quick, multi-agency response to this incident has helped reduce the impact of the oil spill in the area."
She said beach surveys continued and as a precautionary measure booms remained in place to protect the salt marshes around Sandy Haven and the Gann estuary.
Ms Winterton said Valero had already implemented additional measures to minimise any further impact and was co-operating with the investigation.
"We ask people to remain vigilant, avoid any oil if they discover it, particularly if they have dogs," she added.
Valero took over the refinery, which first opened in the mid 1960s, in 2011.
It employs more than 500 workers and has a capacity of 270,000 barrels per day.
In a statement, the company said it "continues to assist the multi-agency response on the Haven" but it would not disclose how many other pipelines it operated.
"Valero's other pipelines and infrastructure remain unaffected and operational capacity remains unaltered."
A spokesman added: "We are continuing to work with agencies to monitor and protect the environment. Valero's top priority remains the safety and the wellbeing of our employees, contractors and communities at all times."
There was an initial spill incident in December when a few birds were affected by oil but not seriously. After the latest spill last week, NRW said a couple of birds were spotted with oil but had not been badly affected.
NRW said anyone aware of any pollution should contact the incident hotline on 03000 65 3000.