UK GAS-Prompt prices mixed, mild weather seen tempering demand

* System undersupplied by 6 mcm/day

* Temperatures seen holding above normal until Friday

April 20 (Reuters) - British prompt gas prices were mixed on Monday as the system was expected to switch from being undersupplied due to milder weather easing demand for heating in the coming days.

Gas for within-day delivery was up 0.3 percent at 45.50 pence per therm, while gas for day-ahead delivery was down 2 percent at 45.30 pence per therm by 0930 GMT.

Meteorologists at Thomson Reuters Point Carbon forecast temperatures holding above normal levels until Friday.

National Grid (LSE: NG.L - news) data showed supply was forecast at 216 million cubic metres (mcm) on Monday, while demand was seen at 222 mcm, showing the system was undersupplied by 6 mcm.

"Tight intra-day supplies are offering some support to within-day, though weaker demand forecasts for the rest of the week could see some downside," said Marcel Boonaert, head gas trader at Wingas UK.

Gas flows from Norway were steady from Friday, with traders expecting a pick up in Norwegian supplies to Britain after maintenance is completed at Troll later this month.

Further along the curve, gas for delivery next month was down 0.7 percent at 44.15 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was down 0.4 percent at 21.35 euros per megawatt-hour.

The benchmark European Union carbon price was trading 0.4 percent higher at 6.91 euros a tonne on ICE Futures Europe. (Reporting by Sarah McFarlane; Editing by Louise Heavens)