LONDON (Reuters) - Former net gas exporter Britain imported more than it produced for the first time in November, with imports increasingly likely to dominate supply as its own output declines, energy network operator National Grid said.

Britain was a net exporter of gas until 2004, but a steady decline in output from the UK Continental Shelf (UKCS) over the last few years has made it more reliant on external suppliers for fuel to heat two thirds of British homes. Imports met 50.8 percent of total demand in Britain last month, surpassing the UK's contribution of 47.5 percent to its monthly gas supply, with fuel held in storage adding 1.7 percent, National Grid said.

"That was the first time ... This year was that milestone of less than 50 percent from UKCS supplies," a spokesman for National Grid said.

"As UK CS supplies decline we will become increasingly dependent on imports."

The opening in May of Europe's largest liquefied natural gas (LNG) import terminal, South Hook, and a smaller nearby facility in Wales have played a major part in boosting imports in 2009.

The terminals, built to make up for declining UK gas output and diversify imports, have helped push down British gas prices.