Army tanker drivers are taking to the roads to deliver supplies to petrol stations hit by Britain's fuel crisis.
About 200 military personnel, half of them drivers, are taking part in Operation Escalin, despite ministers insisting the situation at the pumps is easing.
The troops, who have been on standby since the start of last week, will initially be sent to London and the south-east, where the worst shortages remain.
They include members of 3rd Logistic Support Regiment who have been training with the petrol industry logistics company Hoyers in Thurrock, Essex.
Ministers have been criticised for not sending them out earlier, after a wave of panic-buying led to chaos on the forecourts.
But the government has been using its reserve tanker fleet, driven by civilians, since last week to try to ensure supplies are adequate.
"We are working closely with industry to help increase fuel stocks and there are signs of improvement in average forecourt stocks across the UK, with demand continuing to stabilise," a government spokesman said.
"Stocks in London and the south of England have been recovering at slightly slower rates than other parts of the UK, so we have begun deploying military personnel to boost supply in these areas.
"More than half of those who have completed training to make fuel deliveries are being deployed to terminals serving London and the south-east of England, demonstrating that the sector is allocating drivers to areas most affected in this first phase from Monday."
Operation Escalin was originally designed in preparation for possible fuel shortages after Britain's final withdrawal from the EU.
Speaking about the shortage of HGV drivers blighting the UK industry, Chancellor Rishi Sunak stressed that the problems were not isolated to Britain.
“We’re working very hard to mitigate as much of the challenge here as possible whilst acknowledging these are not issues that are confined solely to the UK,” he told Sky News. “Many countries are seeing the same thing.”
Asked why army drivers were not called in earlier to ease the upheaval, Mr Sunak said ministers had taken action to address problems including increasing driving tests for HGV drivers and introducing temporary visas for overseas drivers.
He said the government would “continue to engage with the industry and different sectors to see if there are other things that we could or should be doing”.
The Petrol Retailers Association has welcomed the military drivers, although it has suggested they will have only limited effect.
Association chairman Brian Madderson said that while the crisis was "virtually over" in Scotland, the North and the Midlands, more than one in five filling stations in London and the south-east were out of fuel.
UK Prime Minister Boris Johnson, attending the opening day of the Tory party conference in Manchester on Sunday, expressed confidence that the crisis was "abating" and said the military was being used as a "precaution".
But Mr Johnson repeatedly refused to rule out shortages in the wider country before Christmas.
As well as an estimated shortfall of 100,000 lorry drivers, businesses from meat producers to retail have warned of empty shelves if the shortages were not addressed.
Mr Johnson acknowledged the country was going through a "period of adjustment" after Brexit, which has cut off the supply of labour from the EU.
He said he was not prepared to resolve the situation by pulling "the big lever marked uncontrolled immigration" to let in more foreign workers.
Mr Johnson said companies should ensure their employees were "decently paid" if they wanted to hire more staff.