LONDON — Faced with the prospect that waves of coronavirus patients struggling to breathe will soon overwhelm its hospitals, the British government issued a cry for help from manufacturers — the builders of automobiles, jet engines, dialysis machines, excavation equipment — to switch their assembly lines over to make mechanical ventilators.

It is a desperate, almost absurd appeal, but the government said it had received hundreds of replies over four days from businesses offering to try.

The call for a “national effort,” issued this week by Health Secretary Matt Hancock, is unprecedented in peacetime and recalls the life-or-death scramble of the late 1930s, when Morris Motors turned its factories from assembling boxy sedans to building Spitfire fighters.

Now, the tool-and-die set is being asked not to repel a blitz of Luftwaffe bombers but to mass-produce stripped-down “simple, functional” mechanical ventilators to push life-sustaining saturated oxygen into the lungs of critically ill patients in intensive-care units.

Britain has a single domestic ventilator manufacturer, Breas, in Stratford-upon-Avon, which has moved to a seven-day workweek to increase production but has a staff of only 150 worldwide, according to the BBC.

Another British-based company, Diamedica, provides simpler ventilators to developing countries. Diamedica Managing Director Robert Neighbour said his firm was ready to help and will have prototypes ready in a few days. “We’ve had very little time to eat, sleep or anything else. We’ve had to ramp up production considerably,” he said.

In terms of speed, he said, “If I have all the parts, one person can put 10 together in a day, but when you’re starting to talk about a shortfall in the U.K. between 20,000 and 30,000 ventilators, that can take a while.”

And so the government has reached out to firms including Rolls-Royce, Airbus, Jaguar Land Rover and Vauxhall.

Simon Stevens, chief executive of NHS England, told a parliamentary committee that Prime Minister Boris Johnson had spoken with manufacturers personally “to seek to bring new supply into the country for mechanical ventilation, and we have set an open-ended goal for what that would be.”

The request reveals a note of panic, as new forecasts predict the number of patients suffering from the coronavirus may soon begin to soar into the tens of thousands here — and that the National Health Service is frighteningly underprepared.

The government this month estimated it had 5,000 ventilators on hand. Stevens provided lawmakers a more precise tally. NHS hospitals in England, which serve a population of 56 million, have only 8,165 mechanical ventilators on hand, including 750 designed to treat children that would have to be repurposed for populations at greater risk, plus another 691 in private practice, and 35 held by the military.

The paucity of ventilators in Britain is made worse by a worldwide shopping spree by governments seeking breathing machines, which can cost between $25,000 to $50,000, depending on the model. Everyone is facing back orders.

British technologists say, in theory, it would be possible for a Rolls-Royce or Mini Cooper factory, with its sophisticated tooling and skilled workers, to turn itself into a ventilator manufacturer.

But can the British boffins DIY the country out of this crisis?

The government is not looking for the most sophisticated ventilator, but an economy model, made with available components. It doesn’t want the Bentley Betayga of ventilators; it wants a Chevy Spark.

The challenges are daunting. 

“There’s the skill and capability in the U.K. to do it. But how quickly?” said Robert Harrison, professor of automation at the University of Warwick.

“These are life-critical, very sophisticated instruments, composed of electronics, sensors, valves, pumps,” he said, adding that the machines are also highly regulated medical devices, not vacuum cleaners.

“The call to produce them is unprecedented and goes back to efforts made in wartime,” Harrison said. “There’s been nothing like this in our lifetimes.”

Tony Hague, chief executive of PP Control and Automation, a 230-person manufacturer with experience making medical devices, listed some of the hurdles.

To make a modern version of a ventilator requires sophisticated electronics. “And all the intelligent components are made in Asia, mostly China, which has had its own problems lately,” Hague said. And new entrants to the modern ventilator business might need licenses from overseas companies that own the designs and other proprietary intellectual property, he said.

As a kind of hack, Hague and others are looking at schematic diagrams of ventilators made 50 years ago. He said that if hospitals could make do with much more basic devices, the kind of electrical, mechanical, pneumatic machines produced in the 1940s, British manufacturers might be able to do that rapidly, with parts off the shelf.

He warned, “they’d look nothing like the units in use today.”

Hague said under normal circumstances, it would be “almost ridiculous” to imagine Jaguar or Rolls-Royce or his own company turning to ventilators overnight, “but this isn’t normal times. You must think differently.”

In the United States, GM and Ford are in talks with the government about possibly supporting the production of ventilators and other medical equipment. Elon Musk, the founder of the Tesla automotive company, tweeted Thursday, “We will make ventilators if there is a shortage.” Many replied that the need was now.

In Britain, Airbus and Rolls-Royce were among those that said they were looking to see what they could practically do to help. Jeremy Townsend, a spokesman for automaker Vauxhall, said: “We are keen to assist at Vauxhall Motor’s Ellesmere Port plant. Once we have more details on the requirements, we will be able to analyze what we can do.”

The government is providing manufacturers with two pages of specifications for a “Rapidly Manufactured Ventilation System,” its stripped-down device.

Requirements include: “Be reliable. It must work continuously without failure (100% duty cycle) for blocks of 14 days — 24 hours a day. If necessary, the machine may be replaced after each block of 14 days x 24 hours a day use.”

But there are doubters.

“The idea that an engineering company can quickly manufacture medical devices, and comply with the rules, is unrealistic, because of the heavy burden of standards and regulations that need to be complied with,” Craig Thompson, head of products at medical equipment maker Penlon, told the BBC.

Ben Fletcher, director of external affairs at Make UK, an engineering trade body, said getting the design right was critical. “It’s got to be a machine that operates in a way doctors are comfortable using,” he said. “There are problems and risks involved with using older technology. It’s not that it can’t do the job, but it’s not the technology people will be comfortable using.”

Fletcher said the government hadn’t given industry a request for a specific quantity of ventilators. “The government has simply said to us, we will buy every machine that can be made.”