Prices of baseload and peakload power for Monday delivery rose sharply on Friday on tightening supply margins, while colder-than-expected temperatures increased power demand for heating, although conditions were seen returning to the seasonal average by the middle of next week, sources said.

At the Platts 11 am London close, the base power day-ahead contract soared to GBP40.75/MWh, gaining GBP5.15 on the day, while peakload posted even stronger day-on-day gains to reach GBP50/MWh, up GBP10.60.

"We've had a little play at it being winter...but it looks mild again by the middle of next week," a trader said.

According to CustomWeather, temperatures in many cities in the UK are expected to plummet below seasonal norms, with London falling 5 degrees Celsius below average Sunday. However, the forecaster expects temperatures to rise to 1 C to 2 C above norms by Tuesday and Wednesday.

In line with weather forecasts, the UK's network operator National Grid expects peak power demand Monday to hit 53.6 GW, but slip to 52.5 GW and 51.3 GW on Tuesday and Wednesday respectively.

The grid also expects surplus margins Monday to narrow significantly to 193 MW, the tightest it has been so far this winter, but improve to 2.6 GW Tuesday.

At midday Friday, UK wind farms supplied 4.4 GW of power to the system, while coal and nuclear power plants generated nearly 8 GW and 7.3 GW of electricity, the grid said.

At the same time, gas-fired power generation stood at 16.5 GW, while French and Dutch imports were steady at nearly 2 GW and 1 GW respectively.