The instructions were given in light of forecasts showing electricity supply margins may be tighter than normal on Monday, said National Grid ESO.
It would be the first time this winter National Grid uses the reserve.
The units are warming up and will be ready to generate electricity early on Monday if needed.
Demand is set to surge during a spell of freezing weather in the UK just as wind speeds drop, curbing supply from windfarms.
The National Grid said it was also activating a live “demand flexibility service”, which allows the ESO more flexibility when demand is at its highest during peak winter days — between 5-6pm on Monday.
The scheme entitles some households with smart meters to discounts if they cut their use of electricity.
It is due to stay in place until March, with 26 energy suppliers including Octopus Energy and EDF signed up, but until now it has only been used in tests.
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National Grid ESO said its announcement should not be interpreted as a sign that electricity supplies are at risk, and “people should not be worried”.
“These are precautionary measures to maintain the buffer of spare capacity we need,” it said on Twitter.
The UK will start to phase out coal next year as it seeks to cut emissions from the power sector.
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But the government asked coal-fired power producers to keep units available this winter as Europe faces a shortage of natural gas. Using coal means less gas is needed to produce power.
Freezing fog is forecast to grip Britain overnight into Monday, sending temperatures dropping. The cold weather is set to last this week. Wind generation will be low until Tuesday.
Britain is the second-biggest market for offshore wind in the world and is particularly exposed to troughs in generation as it relies heavily on expensive gas for backup.
Prices have eased after a run of mild weather allowed Europe to keep more gas in storage than expected.