Britain is preparing to "war game" a gas supply crisis amid rising fears of an emergency in the energy system this winter.
The National Grid is readying critical stress tests with a specific focus on potential shortages caused by Russia's war on Ukraine. Large industrial gas users are already preparing plans to shut down plants and switch to alternative energy sources in such an emergency.
The simulation will take account of ongoing upheaval in energy markets, which is partly driven by speculation that Vladimir Putin will cut off gas flows. It will assess the impact of shortages that industry figures fear are more realistic this year.
One industry source said the war game, an annual event which normally passes unnoticed, has drawn more political scrutiny this year and that winter preparations had picked up pace earlier than usual.
The gas system will simulate a crisis over four days in September and October in an exercise carried out by government, regulators, lobby groups and major energy companies.
The National Grid is also pressing ahead in its negotiations with heavy industry over possible factory shutdowns if energy supplies fall short this winter.
Britain has become more exposed to a shortage of gas supply after Centrica closed the Rough storage site in 2017, with ministers who oversaw the move insisting there was enough "diversity of supply" to guarantee no shortages.
The site off the coast of Yorkshire was the country's largest gas storage facility and the Government has indicated it could provide financial support to allow it to reopen.
The Grid has warned factory owners face energy rationing if they do not accept payments to switch off machinery.
Industrial users of energy have told the Grid they could switch to using diesel to power some sites but that local authorities would need to agree to waive environmental permits.
The Chemical Industry Association has also warned that the number of sites that could operate using back-up electricity generators running on diesel are "limited in number".
Large industrial operators have called for more advanced notice of a gas supply crunch than in previous emergencies. They have said they need at least two or three days' notice rather than a requirement to shut off operations within 24 hours.
Tom Wilcock, a compliance manager at National Grid, will hold a meeting with energy experts in September to explain the nation's emergency plans as part of a "series of assurance and awareness events" ahead of winter. An industry source said that although the emergency exercise had been taking place for decades, it has taken on a particular significance this year.
They said: "We don't know what the exact scenario will be but it will move through the different stages of an emergency.
"Obviously there is a different context to it this year given what we are seeing in Europe and all eyes are on each country's preparations for the forthcoming winter.
"The context this year is that we have a war going on in Ukraine, and that is making people a little more nervous, and it's right that we look at those concerns and make sure we are as prepared as we can be."
Charles McAllister, director of policy at lobby group UK Onshore Oil and Gas, said there would be a "laser-sharp" focus on the outcome of the exercise this year.
He said: "They'll look at a gas deficit warning, a shutdown of industrial gas demand, and fuel switching where they can, although that's a pretty limited option. It's going to be a very interesting exercise this year.
"National Grid knows we consume 70pc more gas in the winter months. This is always a great focus and there is always concern that we might not have enough gas to meet demand, but it's more extreme now. So the focus is greater than it usually would be due to geopolitical, price and supply reasons."
The European Union has told member states to cut gas usage by 15pc until the end of March due to a "recent escalation of disruptions of gas supply from Russia".
Russian gas giant Gazprom has cut deliveries to Europe through its Nord Stream 1 pipeline to just 20pc of capacity.