UK GAS-Prompt prices fall on healthy supply

* Healthy imports lead to slightly oversupplied system

* Lower temperatures forecast for Tuesday

* Withdrawals from storage expected to rise

LONDON, Jan 13 (Reuters) - British prompt natural gas prices declined on Monday morning as healthy gas imports led to a slightly oversupplied system.

Gas for delivery on Tuesday traded at 65.90 pence per therm at 1015 GMT, down 0.85 pence from the previous settlement, while gas for immediate delivery was down 0.90 pence at 65.80 pence.

National Grid (LSE: NG.L - news) data showed demand was at 307.9 million cubic metres on Monday, around 5 mcm below the seasonal norm.

Gas flow was forecast at 311.5 mcm/day, meaning the British gas system was 3.6 mcm oversupplied.

Gas imports from Norway were steady through the Langeled pipeline, while flows from the Netherlands were high at 42 mcm/day.

Temperatures are forecast to decrease by a couple of degrees on Tuesday, which will prompt more demand for gas for heating systems and increase the need to withdraw gas from storage.

Gas withdrawals from storage will be 40 mcm/day on Monday, 10 mcm higher than Friday's level, Thomson Reuters Point Carbon analysts said.

Withdrawals should increase further on Tuesday.

"We expect net withdrawals to increase by 10 mcm/d tomorrow as local (distribution zone) consumption spikes. However, as temperatures get milder the following days, net withdrawals should return to lower levels (around 30 mcm/d)," they added.

Britain has 13 so-called local distribution zones for gas which consist mostly of households, services and small industries.

Further along the curve, gas for delivery in summer this year was down 0.30 pence at 62.95 pence per therm.

In Britain's power market, day-ahead baseload power prices were up 3.30 pounds from the previous settlement at 51.30 pounds per megawatt-hour.

EDF Energy's Heysham 1-1 nuclear unit went offline in an unplanned outage on Friday evening, around an hour after it had restarted after a previous outage.

There is currently 2,260 megawatts of nuclear capacity offline. (Reporting by Nina Chestney; Editing by Anthony Barker)