The terminal - a joint venture between US major Exxon Mobil, state-run Qatar Petroleum and French energy group Total - can pump gas into the national grid at a rate of about 11 billion cubic metres (bcm) a year.
It will be able to supply 21 bcm a year of gas once the second phase of the project is complete and the last of its five storage tanks enter service by the end of 2009, its project manager said.
"The commissioning of the plant is done," Daniel Wieczynski told journalists at the inauguration on Tuesday of the terminal in south Wales, which was was attended by the Emir of Qatar and Britain's Queen Elizabeth II.
Two of the storage tanks, each the size of London's Albert Hall concert venue, are already operational. The third is expected to come into service in June, the fourth in October and the fifth before the end of the year, Wieczynski added.
Liquefied Natural Gas (LNG) was embraced as a fuel for the future because in contrast to pipeline gas, it can be shipped anywhere in the world.
But the infrastructure required is very expensive and demand has fallen significantly because of global economic slowdown.
The chief executive of Qatargas predicted consumption of the fuel should begin to recover from early next year.
"Yes, demand has gone down and we see more production coming, this will put pressure on prices. But we planned those projects on lower prices than we see in the market now," Faisal Mohammed Al-Suwaidi told reporters.
Britain's annual gas demand stands at about 100 billion cubic metres, according to figures provided by British gas and electricity supplier Centrica.
Following the decline of the nation's oil and gas production in the North Sea, about half of its needs are expected to come from imports this year. By 2017, LNG is expected to account for nearly 30 percent of Britain's total supply.
(EurActiv with Reuters.)