UK GAS-Up on tighter domestic supply, reduced Norwegian imports

* Prices rise across the board on tight system

* Exports to Europe expected to slow, more storage needed

March 25 (Reuters) - British wholesale natural gas prices extended gains on Wednesday as the network struggled to meet demand with lower Norwegian supplies and reduced output from domestic storage sites.

Prices for next-day delivery rose 0.25 pence per therm at 47.60 pence by 0953 GMT, while the same-day contract notched a 0.15 pence gain to 47.65 pence per therm.

The gas network was undersupplied by around 11 million cubic metres/day with total daily demand pegged at 288.7 mcm by National Grid (LSE: NG.L - news) , around 18 percent above seasonal norms.

Offshore maintenance affected deliveries into the grid via the Bacton Seal terminal, operator Shell (LSE: RDSB.L - news) said on Tuesday. Flows remained under pressure at just below 10 mcm on Wednesday, compared with 13 mcm earlier in the week.

A Shell spokeswoman declined to comment on the duration of maintenance.

Imports via Norway's Langeled pipeline into Britain dropped to around 45 mcm from 50 mcm overnight, National Grid data showed.

"We have seen volatile Langeled flows over the past couple of weeks. As we expect a lower rate in April, we could see a progressive transition to this lower level until then," analysts at Thomson Reuters Point Carbon said in a note.

Throughput from the two Teesside terminals edged higher in tandem, while Theddlethorpe flows faltered early on Wednesday. A spokeswoman for the terminal's operator ConocoPhillips (NYSE: COP - news) declined to comment on operational issues.

An unplanned outage at Britain's Aldbrough gas storage site has been extended to April 4, operator SSE (LSE: SSE.L - news) said on Wednesday.

Withdrawals from gas storage sites declined overall as Rough, Britain's biggest such facility, plunged to slightly above 5 mcm from 33 mcm in the previous session.

To compensate, the mid-size Holford facility ramped up output to 22 mcm from zero on Tuesday, National Grid data showed.

"Further storage supplies are likely going to be needed to fill the current supply gap should import levels not pick up by later in the session, though prompt price spreads will have to widen to offer incentive for supplies to start," Marcel Boonaert, head trader at Wingas UK.

Gas exports from Britain to Belgium via the undersea gas link are expected to reduce later this week as consumption demand in mainland Europe tapers off, analysts said.

This should lead to higher levels of supply in Britain.

In the Netherlands, the day-ahead gas price at the TTF hub fell 0.20 euros per megawatt-hour (MWh) to 21.50 euros.

The benchmark European Union carbon price fell 0.56 percent at 7.04 euros a tonne. (Reporting by Oleg Vukmanovic; Editing by William Hardy)