London (AFP) - Britain's electricity grid has emergency plans to pay factories to shut down during peak hours if it comes under strain this winter, a report said Tuesday.

The National Grid will have the narrowest spare capacity in seven years this winter, close to 4 percent compared to 5 percent last year.

Unusually bad weather and Russia cutting off gas supplies due to tensions with Ukraine would be an "extreme scenario" that might require shutdowns and imports from Asia or South America, the report said.

"The electricity margin has decreased compared to recent years, but the outlook remains manageable and well within the reliability standard set by government," National Grid director of market operations Cordi O'Hara said.

"As system operator, we have taken the sensible precaution to secure additional tools to bolster our response to tighter margins."

Earlier this year, the operator signed deals with 431 industrial sites to pay them to shut down at peak hours if needed.

The National Grid winter outlook said there was a one-in-31 risk of emergency measures being needed.

Electricity supplies have been squeezed as fires have caused the partial and temporary shutdown of two power stations and another to permanently close.

Another aging power station is set to close in London, while several nuclear reactors are returning to service at reduced capacity.