The longest continuous coal-free period in the United Kingdom was 19 hours in May 2016 and matched this Thursday.
The grid said it would only know for sure if it had achieved the feat by "22:51 on Friday".
Coal has seen significant declines in recent years, accounting for just 9 percent of electricity generation in 2016, down from around 23 percent the year before.
Last year, the government said it wanted to phase out Britain's last plants by 2025 as part of efforts to cut carbon emissions.
If Britain goes coal-less Friday, it will be the first time since the world's first centralized public coal-fired generator opened at London's Holborn Viaduct in 1882.
Environmental campaigners are celebrating after the grid operator said below-average electricity demand and a good output from wind and solar farms meant that coal was extremely unlikely to be needed to produce energy today.
Image copyright Getty Images Image caption The Clean Air Act was in introduced in 1956 to tackle the smog coal fired stations created by in big cities.
Here's your rodeo weather forecast for the north valley
Showers are likely throughout the mid-valley on Thursday , and forecasters say there's a chance of thunderstorms. Heavy rain is falling in southern Missouri and will continue to drift to the north as the day progresses.
Coal has powered Britain for more than a century.
"A decade ago, a day without coal would have been unimaginable, and in 10 years' time our energy system would have radically transformed again". They expected it to be a full day today, they reported on Twitter.
"We then had the dash for gas in the 1990s, with nuclear roughly contributing around 25%, and coal dropped below 50%".
The UK has had shorter coal-free periods in 2016, as gas and renewables such as wind and solar play an increasing role in the power mix.
National Grid said the record low is a sign of things to come, with coal-free days becoming increasingly common as the polluting fuel is phased out. "A decade ago, a day without coal would have been unimaginable", said Hannah Martin, head of energy at Greenpeace UK.
Fiddlers Ferry coal fired power station near Liverpool. "But we will see the end of United Kingdom coal before the 2025 deadline and the Government must pave the way for the transformation".
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