National Grid has moved to counter fears of blackouts this winter by insisting that electricity margins are "manageable".
In its winter outlook, the energy company confirmed predictions earlier in the year which showed the gap between total electricity generating capacity and peak demand would fall to just 1.2% without measures in place to secure supplies.
The measures National Grid has put in to balance the system, such as paying moth-balled power plants to be ready to come online and paying factories to be prepared to power down if needed, increase the capacity margin to 5.1%.
Cordi O'Hara, director of UK market operations said: "Electricity margins are manageable throughout the winter period and we believe we have the right tools in place to manage the system.
"This includes using the 2.4 gigawatt of additional balancing services that we have ready in place for times of highest demand.
"On the gas side, supplies are expected to be comfortable this year, thanks to good availability of liquefied natural gas on the global market and stable flows from the North Sea and Norway."
National Grid said gas demand for this winter was expected to be broadly in line with last year, showing a slight increase to 48.6 billion cubic metres, with peak daily demand forecast to be 465 million cubic metres.
The maximum potential delivery of gas supplies, including from storage, is 613 million cubic metres - significantly higher than expected peak demand, National Grid said.
For electricity, demand is expected to peak in mid-December, while the weeks commencing October 26 and January 11 are predicted to have the lowest power surplus due to planned outages, the winter outlook said.
But the report said there was expected to be "sufficient" generation and imports of electricity through connections to other countries to meet even the tightest weeks over the winter.
Buying the extra breathing room of power plants on standby and factories that are prepared to power down has cost £ 34.7 million and will add less than 50p to the average consumer bill.
Union leaders accused the Government and National Grid of being "far too complacent" about the risks of widespread power blackouts.
Brian Strutton, national officer of the GMB union, said: "There can be eight to 10 days per month when there is not a lot of output from the 10.2 GW of installed wind capacity.
"We have the bonkers position where National Grid is using consumers' money to pay firms to stop work in order to avoid winter blackouts. National Grid hide this farce with the phrase "energy balancing".
"That and bringing unused inefficient power production back into operation are the special measures National Grid is being forced to rely on to keep the lights on and the cost is added to consumers' bills.
"National Grid and Ofgem should come clean about the true cost of this harum scarem energy balancing game. The Government has got to urgently get a grip and ensure there is sufficient energy production to securely provide the nation's energy needs."
But Richard Black, director of the Energy and Climate Intelligence Unit, which published said: "The fact is that generation-related electricity outages are vanishingly rare - just one in the last 10 years - almost all power cuts are down to problems with local distribution of electricity, caused by bad weather and other issues."
He added: "Unfounded warnings about security of supply simply serve to distract from the other serious debates we need to have about energy policy, such as how we cut greenhouse gas emissions whilst keeping people's bills manageable."
Fears have been raised that electricity supplies have tightened significantly as a number of power plants close, in particular coal fired power stations which are shutting as they do not meet environmental standards.
New nuclear sites such as one planned in Hinkley, Somerset, are not set to start generating power for years, while much of the new generation coming on line is from renewables such as wind and solar which can be intermittent.
Energy and Climate Change Secretary Amber Rudd said: "Keeping the lights on is non-negotiable. National Grid has the right tools in place to manage the system this winter and we will ensure that they continue to do so in future."
She said the number one priority was to ensure consumers and businesses had access to secure, affordable and reliable energy supplies, and in the longer term the Government was investing in infrastructure and low-carbon energy supplies to improve energy security for future generations.
Doug Parr, chief scientist of Greenpeace, said: "Every year the National Grid say they can keep the lights on, but every winter, some still say our energy infrastructure will struggle to cope with a cold snap, alongside energy firms knocking on the Government's door for more handouts.
"Sustainable long-term security means that George Osborne needs to empower cities and towns to provide local, decentralised energy which can help provide the resilience that the country needs, together with a smart interconnected grid and clean energy."
Shadow energy and climate change secretary Lisa Nandy said: "The constant chopping and changing of energy policies under the Tories has put off urgently needed investment in new, clean power stations.
"Already energy investors have begun to turn their backs on Britain and we now know it is also damaging Britain's energy security.
"Today's analysis from National Grid shows we now have less spare power capacity than at any time for almost a decade.
"This is a situation that could have been entirely avoided with a more stable energy policy and without the Chancellor's near constant attacks on clean energy investment."