LONDON Feb 27 (Reuters) - British spot gas prices moved higher on Monday morning as traders reacted to a structural shift in the market by topping up stockpiles at underground sites, which made the network undersupplied. The prompt market saw gains of over a penny with gas for immediate delivery trading at 61.20 pence per therm and Tuesday gas at 61 pence. A spell of unseasonably mild weather has depressed near-term gas prices relative to later-dated contracts in summer 2012, for example, making it more profitable to store gas now with a view to selling it at higher prices later. Normally, gas traders store volumes in summer and withdraw when prices and demand are high in winter. The summer gas contract on Monday traded 0.30 pence above gas for immediate delivery at 61.5 pence per therm. As a result, the UK's biggest underground storage site, Rough, switched from supplying the country with gas to draining volumes from the grid. "High oil prices and a lack of liquefied natural gas (LNG) deliveries are providing support to the summer gas contract," one trader from a major European utility said, explaining why forward contracts are rising at a faster rate compared with near-term prices. WEATHER Temperatures are expected to stay mild throughout the week and beyond, according to the UK's Met Office. "Temperatures are likely to be often close to or just above average with a chance of some slightly colder interludes later," it said in its 6-15 day forecast. DEMAND AND SUPPLY The gas network was about 7 million cubic meters undersupplied on Monday morning, despite demand running about 14 percent below seasonal average levels. POWER Day-ahead baseload power for 24-hour delivery traded at 43.6 pounds per megawatt hour. (Reporting by Oleg Vukmanovic; Editing by Alison Birrane)

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