* Gas system undersupplied by 3 mcm/day
* Easington maintenance to start on Tuesday
April 18 (Reuters) - British prompt gas prices fell on Monday morning on an expected increase in flows from the UK Continental Shelf and a drop in oil prices after producers failed to agree on an output freeze in Doha on Sunday.
Gas for immediate delivery fell by 0.8 pence or 2.76 percent to 28.20 pence per therm by 0835 GMT, while the day-ahead contract lost 0.05 pence or 0.18 percent to 28.15 pence/therm.
With (Other OTC: WWTH - news) supply flows forecast to be around 251 million cubic metres (mcm) and demand seen at 254 mcm/day, Britain's gas system was expected to be 3 mcm undersupplied, National Grid (LSE: NG.L - news) data showed.
Despite the system being slightly undersupplied, flows at Total (Euronext: FP.NX - news) 's St Fergus terminal were expected to ramp up on Monday after an outage had cut output by 7.4 mcm since Friday, while supplies from Norway also rose ahead of maintenance at the Easington terminal.
"Maintenance at Easington is offering prompt contracts support.. The confirmation of an additional LNG shipment into South Hook for April 22 is potentially limiting upside," a UK-based trader said.
The Mozah tanker is expected to arrive at South Hook on Tuesday and another tanker, Al Khattiya is due to arrive on Friday.
The Easington terminal maintenance is expected to reduce import capacity by 36 mcm per day from Tuesday until May 3, its operator Gassco said.
Maximum temperatures in London and South East England are forecast to rise to 13 degrees Celsius on Tuesday from 12 degrees Monday, UK's Met Office said, potentially reducing demand for heating.
However, temperatures are forecast to fall from Friday, and cooler, more unsettled weather is expected to persist through the start of May, the Met Office added.
Meanwhile, oil prices tumbled on Monday after a meeting by major exporters in Qatar collapsed without an agreement to freeze output, which put pressure on gas contracts further out on the curve.
Contract for gas delivery next winter eased 0.2 pence to 32.25 pence/therm.
In the Dutch gas market, the day-ahead price at the TTF hub rose 0.01 euro/MWh to 11.28 euro/MWh. In the European carbon market, front-year allowances eased 0.07 euro to 5.41 euro a tonne. (Reporting by Nerijus Adomaitis; editing by Nina Chestney)