Two old coal-fired power plants in the UK began generating again yesterday as the country was blanketed in snow and ice.
They had been put on standby in case of shortfalls - and started feeding power into the grid in the afternoon.
National Grid blamed high demand and a shortage of electricity from other sources.
The coal plants began operating in 1966, but were due to close last September.
However, operators have kept them open for an extra six months at the request of the government, amid fears of possible power shortages.
The two coal-powered stations in use again are in West Burton in Nottinghamshire.
Stood down
Two further contingency coal fired units at Drax power station in Yorkshire were ordered to get ready for use yesterday, but were subsequently stood down.
National Grid has a total of five coal-fired generating units on standby - the other is at Ratcliffe on Soar.
The extra generating capacity was lined up this week due to concerns that the margin between supply and demand was becoming too narrow.
The BBC says National Grid blames the cold weather, a shortage of wind and solar power generation, and a lack of electricity available through interconnectors from France due to strike action in the country.
However, plans to use the so-called demand-flexibility service - which pays people to use less electricity - have been set aside.
Although the current cold weather is expected to continue, the grid is expected to be under less strain because more electricity will be available through international undersea cables.
Emissions
Coal is the dirtiest of all fossil fuels and produces almost twice the emissions of natural gas.
Gas and wind power have been gradually replacing coal in powering the UK's energy supply over recent years.
But this has been delayed by the UK Government following concerns over the disruption to Russian gas supplies to Europe after the invasion of Ukraine.
Coal-fired power generation accounted for just 2% of the country's electricity produced last year.