* Record gas demand od 465.8 mln mcm expected
* Power demand also close to record, at 59.5 GW
* But supply stable says Nat. Grid
(adds power demand, and detail)
By Henning Gloysstein
LONDON, Dec 20 (Reuters) - Gas demand across Britain is expected to hit a record 465.8 million cubic metres (mcm) on Monday, causing National Grid to issue its first gas balancing alert this winter on Sunday evening, the operator said on Monday.
The highest demand so far this winter was 456.6 mcm on 2 December, and the current all-time record is 465.5 mcm set on 8 January 2010, the grid operator said.
"National Grid issued the first Gas Balancing Alert of the winter yesterday as the gas demand forecast for Monday was above the current trigger level of 452 mcm," National Grid said.
Despite this warning, National Grid said that healthy levels of electricity generation and gas available from a diverse range of sources has left supply and demand balanced despite current high demand levels due to the cold weather.
"Increased demand for energy is an inevitable consequence of the cold weather as Britain shivers. However, we remain well supplied with gas and electricity, thanks to strong availability from a diverse range of sources," Chris Train, National Grid's Network Operations Director said.
The cold weather is also lifting electricity demand in Britain.
"Electricity demand for Monday is forecast to peak at 59.5 Gigawatts (GW). The highest level of demand so far this winter was 60.0 GW on 7 December 2010, and the current all-time record is 61.0 GW set on 10 December 2002," National Grid said, but added that supply supply this winter seemed safe.
"There continues to be a healthy surplus of generation capacity available. The analysis in National Grid's Winter Outlook Report indicated that the surplus of generation over demand was expected to be higher than in recent years," the operator said.
British power demand is less affected by cold weather than as the gas market because most British households are gas heated.
National Grid also said that "demand for gas and electricity is expected to fall from the middle of the week as the usual drop in usage over the Christmas holidays begins."
The operator said that "availability of gas supplies so far this winter has been higher than anticipated in National Grid's Winter Outlook Report, notably through the interconnector with Belgium and liquefied natural gas (LNG) imported by tanker."
Britain's ability to import LNG received a recent boost from the expansion of National Grid's Grain LNG facility, which entered full commercial operation on 1 December and now has the capacity to meet 20 percent of Britain's annual gas demand. Storage remains nearly two thirds full despite an earlier start to withdrawals this winter because of the cold weather, according to National Grid.
"Many storage facilities have alternated between withdrawing gas on colder days and injecting gas back into storage on days of lower demand since the freezing weather began in November."
This is the sixth gas balancing alert to be issued since the tool was introduced in 2005, according to the grid operator. (Editing by XXX)