UK GAS-Prices fall to seven-year low on heavy oversupply

* System oversupplied by 17 mcm

* Within-day, day-ahead fall to lowest levels since 2009 (Updates prices, adds new comment)

Aug 31 (Reuters) - British prompt gas prices plummeted to seven-year lows on Wednesday afternoon as no more gas could be taken out of the heavily oversupplied system.

Gas for immediate delivery plunged by over 15 percent to 22.35 pence per therm at 1336 GMT while the day-ahead price fell by 12 percent to 23.55 pence per therm.

Both contracts are at their lowest levels since 2009.

The gas system was oversupplied by around 17 million cubic metres, National Grid (LSE: NG.L - news) data shows, but Britain's export pipeline InterconnectorUK was at its maximum capacity.

Storage levels were also very high with little room left for gas to be injected to get it out of the system.

"It's all on supply - flows have increased but there's nowhere left for it to go," a UK gas trader said.

Exports to Europe were already running at a record high in early trade, according to flow data, with the strong flows from the UK Continental Shelf putting pressure on prices. (Reporting by Nina Chestney and; Oleg Vukmanovic; Editing by Greg Mahlich)