Troops in camouflaged fatigues were seen at Buncefield Oil Depot in Hemel Hempstead 20 miles (32 kilometres) north of London after the military was put on standby last week.

Some 200 military personnel, half of them drivers, are taking part in Operation Escalin to alleviate fuel shortages in London and southeast England worst affected by the run on the pumps.

The Petrol Retailers Association (PRA), which represents 65 percent of Britain's 8,380 total independent forecourts, welcomed the intervention of the military.

But it warned that soldiers were still likely to have only a limited effect.

One in five filling stations in London and southeast England were out of fuel, while the crisis in the rest of the country was "virtually over", PRA chairman Brian Madderson said.

Britain's roads have been clogged by long queues of motorists at petrol stations for more than two weeks.

Frustrations have boiled over into anger, and violence has even broken out on some forecourts between motorists desperate to fill up, including with jerrycans and even old water bottles.

Critics have blamed the chaos on Britain's exit from the European Union, coronavirus and a lack of government planning to replace thousands of foreign drivers who have left the country.

However, Prime Minister Boris Johnson said Britain's supply chain issues were a "function of the world economy, particularly the UK economy, coming back to life after Covid".

Poland, the United States and China were also experiencing driver shortages, he said.

- No 'magic wand' -

The government has made a U-turn on its tougher, post-Brexit immigration policy, relaxing curbs to give short-term visas to 5,000 foreign lorry drivers and 5,000 poultry workers to help plug staffing gaps.

But Johnson is resisting any further easing, saying he wants to see a "high-wage, high-skill economy" rather than mass immigration which would drive down salaries.

He called for the road haulage industry to invest in drivers' wages to make jobs in the sector more attractive.

Finance minister Rishi Sunak told the BBC that the government was doing everything it could to address the fuel crisis and food shortages also attributed to driver shortfalls and vacancies in the meat processing sector.

That has raised fears of a shortage of turkeys for Christmas.

Pig farmers protested outside the annual conference of Johnson's ruling Conservative party in Manchester, northwest England, on Monday, to highlight a lack of skilled butchers.

They said the lack of butchers and abattoir workers -- many from overseas -- could see up to 120,000 animals slaughtered and incinerated, rather than going into the food chain.

"Do you really want to see this again?" one poster read, showing a photograph of burning pyres of cows slaughtered during the "mad cow" crisis of 2001.

Sunak also argued that wider issues were to blame that were affecting other countries, not just Britain.

"There are things that we can do and should do and it is reasonable that people expect us to do what we can," he said.

"But we can't wave a magic wand and make global supply chain challenges disappear overnight," he said, warning of further shortages in shops into the winter months.