Electricity margin notices are aimed at encouraging generators to make any extra capacity available, with the prospect of being paid premium rates to do so.
Neso said the notices do not signal imminent blackouts or that there is not enough generation to meet demand.
Margin notices have only previously been needed in winter, when demand for heating and lighting surges.
However, as the UK becomes increasingly reliant on weather-dependent renewables, such as wind and solar, extra capacity is essential in summer as well.
A Neso spokesman said: “Our forecasts indicate tight electricity margins. This is due to extreme temperatures across Europe, reducing the availability of some generation.”
Kathryn Porter, an energy analyst, said policy makers were increasingly gambling on the weather and on other countries to keep Britain’s lights on.
“This is only going to get worse,” she said. “High pressure weather systems with lots of sun but little wind are common both in summer when they cause high temperatures and winter when they bring the coldest temperatures.
“Clearly we’re struggling to manage. We need sensible energy policies and to stop gambling on the weather and praying other countries can bail us out.”
The heat is expected to peak on Thursday and Friday when temperatures will “widely exceed 30C” across the UK, according to the Met Office.
