A controversial £1bn pipeline capable of carrying a fifth of the natural gas needed in Britain has been completed from Pembrokeshire to Gloucestershire.
Campaigners staged protests along sections of the route
UK Energy Minister Malcolm Wicks marked the occasion by opening a valve into the final section of the line at Felindre, near Swansea.
The National Grid says the scheme, which has taken three years to finish, is needed to cope with growing demand.
But campaigners have protested on safety and environmental grounds.
The fuel will be shipped in as liquid to two terminals at Milford Haven, where it will be converted back into gas and be transported along 196 miles (316km) of pipeline.
Specially-built giant tankers used to ship the gas to Britain are set to start arriving at the port next year.
Construction of the pipe was hit by protesters at Trebanos, in the Swansea Valley, and at woodland just outside Brecon where contractors were forced to obtain an eviction notice to move them.
Residents at Cilfrew near Neath went to the High Court in a bid to overturn planning permission for a sub-station near their village.
The pipeline runs from Milford to Tirley. En route, it makes 140 river and water crossings, passes under 19 railway lines and 216 roads - two of them motorways.
The project has affected 833 landowners and has resulted in two significant archaeological finds - a Roman road near Yscir, west of Brecon, and a possible Bronze Age canoe near Milford.
National Grid senior project manager David Mercer said completing the pipeline had been "a massive undertaking through some very difficult countryside".
He added: "Everyone who has worked on the project can be immensely proud - it's a tremendous engineering achievement.
"This pipeline has been built to the highest internationally recognised standards.
"I would also say this pipeline is very important for Wales - it puts Wales at the front of gas supply in the UK and it creates many opportunities in west and south Wales for development."
But some critics have questioned the need for the pipeline and say claims over the amount of gas it will contribute to the UK network have been greatly exaggerated.
Paul Sinadurai, senior ecology and policy advisor for the Brecon Beacons National Park, which was among objectors to the pipeline, said: "It kind of leaves a bitter taste in the mouth.
"It's always very easy to grass over a piece of land that has been ploughed previously.
"But where you cut through ancient woodlands or where you have gone across internationally important river systems.. (we and others) are not convinced there will not be a significantly adverse affect."