Campaigners against a £1bn natural gas pipeline who petitioned the European Parliament have been told the matter will be investigated.
They had met MEPs to outline their safety concerns over the 196 mile (316km) pipe and the liquefied natural gas (LNG) terminals in Milford Haven.
The European Commission will examine the case and report back to MEPs.
National Grid said it believed it had complied with the relevant EU consent and construction directives.
The company, which was at the European Parliament to put its case, said a full environmental impact assessment required by EU law had been carried out, as well as extensive public consultation.
The European Commission will report back to the European Parliament Petitions' Committee on whether it feels EU directives have been breached.
Plaid Cymru MEP Jill Evans, who accompanied the campaigners on Thursday, said if evidence was found to support the campaigners' case, the commission could take action against the UK government.
"That usually starts with a letter, which sounds pretty weak, but actually that first notice letter is very serious and it's often enough in itself to get things moving," Ms Evans said.
'Growing demand'
The pipeline, from Pembrokeshire to Gloucestershire, opened in November.
National Grid has said the scheme, which took three years to complete, was needed to cope with growing demand for gas in Britain.
The fuel will be shipped in as liquid to two terminals at Milford Haven, where it will be converted back into gas to be transported along the pipeline.
Specially-built giant tankers, used to ship the gas to Britain, are due to start arriving at the port in 2008..
The pipeline's construction was hit by protests at Trebanos, in the Swansea Valley, and at woodland just outside Brecon, where contractors obtained an eviction notice to move them.
Residents at Cilfrew near Neath went to the High Court in an attempt to overturn planning permission for a sub-station near their village.
National Grid responded to the issues raised by the campaigners on Thursday in a statement which said: "We believe that we have complied with the relevant EC directives through the consenting and construction processes for the new pipelines from Milford Haven to Tirley."
A Welsh Assembly Government spokesman said new gas supplies to the UK were essential to ensure security of energy supply.
He said: "We have been assured by National Grid Transco (NGT) that the new LNG pipeline is being built to the highest safety standards. The Health and Safety Executive has also been closely involved in the construction of the LNG project.
"National Grid Transco have also committed to providing the highest level of land restoration for the LNG pipeline and have a good reputation in this area."
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