* Gas system undersupplied by 7.5 mcm
* Within-day prices up to 50.40 pence per therm
LONDON, Sept 16 (Reuters) - Natural gas prices in Britainrose on Tuesday morning as maintenance at Shell's St Fergusterminal contributed to an undersupplied system.
Prices for within-day delivery were trading at 50.40 penceper therm at 0909 GMT, up 1.40 pence since their lastsettlement. Prices for delivery on Wednesday were up 1.15 penceat 50.00 pence per therm.
With supply flows at about 168.9 million cubic metres (mcm)per day on Tuesday and demand expected to be about 176.4 mcm,the system was 7.5 mcm undersupplied, National Grid data showed.
The system was short partly due to a reduction in volumesthrough Shell's St Fergus processing terminal.
National Grid data showed volumes fell to zero early onTuesday morning from more than 20 mcm/day on Monday evening.
Shell said on its website it extended a 12 mcm/day capacityreduction at St Fergus until the week commencing Sept. 29. Aspokeswoman declined to comment on the discrepancy in the lossof volume.
Meanwhile ConocoPhillips' Southern Area Facilitiesgas fields are undergoing maintenance, which has reducedreceipts of gas to the Theddlethorpe terminal by around 10 mcm.
Further along the curve, gas for November delivery was up0.20 pence at 59.75 pence per therm and the Winter 2014 contractwas up 0.10 pence at 60.85 pence.
Traders said curve prices were buoyed by ongoing fears aboutthe delivery of Russian gas via Ukraine to Europe, as somecountries, such as Slovakia, have received less gas than usualthis week.
Nominations for Russian gas entering Slovakia from Ukrainewere set at 49.6 mcm for a second day in a row on Tuesday, belowdaily amounts seen last week, data from Slovak pipeline operatorEustream showed. (Reporting by Susanna Twidale; editing by Jason Neely)
