UK GAS-Gas prices rise as colder weather increases demand

* System undersupplied by 6 mcm/day

* Temperatures seen dropping until Friday

LONDON, April 21 (Reuters) - British gas prices inched up on Tuesday as demand stabilised after temperatures fell again following last week's mild weather.

Gas for within-day delivery was up 0.77 percent at 46.00 pence per therm by 0830 GMT, while gas for day-ahead delivery was up 0.33 percent at 45.85 pence per therm.

Britain's Met Office forecast temperatures would be between 7 to 10 degrees on Tuesday across the country, with the skies expected to be cloudier across parts of northern and eastern Britain at night.

National Grid (LSE: NG.L - news) data showed supply was forecast at 203 million cubic metres (mcm) on Tuesday, while demand was seen at 209 mcm, showing the system was undersupplied by 6 mcm.

"We are seeing stable demand and the temperature outlook is a little colder," a gas trader said.

Analysts at Thomson Reuters Point Carbon said a switch from withdrawals to injections of gas at Britain's main gas storage site Rough also provided support for prices.

"Rough is likely to continue in injection mode from now on, which adds to demand," the analysts said in a daily report on Tuesday.

Gas is typically withdrawn from storage sites during the winter months when demand is higher and injected into storage over spring and summer when temperatures are higher.

Centrica (LSE: CNA.L - news) owner Rough accounts for around 80 percent of Britain's gas storage capacity.

Further along the curve, gas for delivery next month was up 0.11 percent at 44.70 pence per therm.

In the Netherlands, the day-ahead gas price at the TTF hub was down 0.6 percent at 21.67 euros per megawatt-hour.

The benchmark European Union carbon price was trading 0.7 percent lower at 7.12 euros a tonne on ICE Futures Europe. (Reporting by Ahmed Aboulenein and Susanna Twidale, editing by Louise Heavens)