UK GAS-Prices firm, supported by low storage levels

* System 14 mcm long

* Rough injection capacity returns after outage

Aug 4 (Reuters) - British prompt gas prices were firm on Tuesday, supported by low storage levels and the resumption of injection capacity at the largest storage site Rough after an unplanned outage.

Gas for day-ahead delivery rose 0.35 pence to 42.15 pence per therm by 0911 GMT.

Britain was oversupplied by 14 million cubic metres (mcm), with demand estimated in line with seasonal average levels at 149 mcm, while flows were 163 mcm.

Prompt prices were expected to remain supported by lower storage volumes than the previous year.

"The storage injections are likely to keep demand good and, given this, prompt prices should remain in range over the week," Energy Aspects said in a note.

"Fundamental resistance to the downside is likely to be strong at 40 pence per therm."

According to Energy Aspects, European gas storage is 22 percent below levels seen at the same time last year, at 15.2 billion cubic metres (bcm).

British utility Centrica (LSE: CNA.L - news) 's Rough gas storage site saw injection capacity return on Tuesday after an outage earlier in the week.

Weather is forecast to remain in line with seasonal norms in the coming days.

Further along the curve, gas for September delivery was 0.03 pence higher at 41.68 pence per therm.

In the Dutch gas market, August gas at the TTF hub edged up 0.05 euros to 20.38 euros per megawatt hour, as the Dutch gas system was largely balanced.

In the European Union's carbon market, the benchmark price for December delivery rose 0.01 euros to 7.97 euros a tonne. (Reporting by Sarah McFarlane; editing by David Evans)