LONDON — With the country basking in a glorious heat wave and England cruising through to the knockout stages of the soccer World Cup, most people wouldn’t begrudge Londoners for quenching their thirst with a cool pint of lager.

However, the city’s soccer enthusiasts, pub owners and sun-seekers are currently facing a big problem: Beer supplies are running low as Northern Europe, and the U.K. in particular, experiences a critical shortage of CO2 — or carbon dioxide — used to put fizz in drinks.

This week, several of the country’s biggest pub chains and beer brands acknowledged they were facing serious supply difficulties, and a major wholesaler announced it was rationing beer and cider. Bottling companies also said they’d been forced to stop production.

With London’s bars currently festooned with flags from around the world, fans attending screenings of Wednesday’s big World Cup games told NBC News they couldn’t imagine watching this year's tournament without beer.

“I mean, most people watch the World Cup with alcohol, right?” Nathan Rutsaert, an 18-year-old from Brussels, said while watching Germany’s match with South Korea at east London’s Bavarian Beerhouse. “And beer’s the main one.”

“Especially for Germans!” said friend Annabel Hascher, another 18-year-old student, as she proudly wore her team’s jersey. (Eighteen is the legal drinking age in the U.K.)

England supporters celebrate Harry Kane's winning goal as fans watch the World Cup soccer match between Tunisia and England at the Lord Raglan Pub in London on June 18, 2018. Nigel French / AP

Germany lost its match, knocking it out of the tournament. England did, too, 1-0 to Belgium, on Thursday night, but still moved on, much to Londoners' delight.

John Raquet, the CEO and founder of industry publication Gasworld, said that simultaneous outages and seasonal maintenance closures in CO2 plants across the U.K. and northern Europe had caused huge supply problems.

British plants were running at 20 percent capacity, compared to the usual seasonal average of 70 percent, he said.

“On top of that, there was the heat wave in Northern Europe in May that increased demand for soft drinks, alcoholic drinks and food. So you had higher-than-expected demand and a lower availability of CO2,” Raquet said.

Shifting demands in the farming industry for industrial ammonia — of which carbon dioxide is a by-product — had also contributed to the shortage, he added.

Whatever the cause, a scarcity of beer in a brew-loving nation is no casual thing, and England fans expressed worries ahead of their next game.

"Beer shortage? There will be riots in the streets," said 23-year-old Alex Mawhinney. "It will be a nightmare."

John Miller, a 25-year-old city planner, said not having beer with his soccer was "pretty unfathomable to think about it."

"How I would deal? I would just probably sleep a little more," he said.

Despite the shortage, London is far from a beer-free zone. Imported pilsner was flowing freely into the steins of the Bavarian Beerhouse, and some breweries don’t buy in CO2 during their production processes.

However, staff at the nearby Roadtrip bar said they had issues with CO2 suppliers. With fans glued to the many screens on the walls, around half of the beer pumps had plastic cups on top, showing several popular brands to be unavailable.

A heat wave hit London this week, with temperatures in St. James's Park in the mid-80s F. Tolga Akmen / AFP - Getty Images

They had enough CO2 in stock, though, to continue pumping beer through their taps.

“It’s not hit the taps yet,” the bar’s manager, Maria Gunnarsson, said shortly after her team (Sweden) scored its second goal against Mexico. “But obviously the supplier can’t send us all the beers.”

“It’s mainly Amstel,” her colleague Serge Vicente interjected from the other side of the bar. “And there’s been problems with cider.”

And it’s not just Roadtrip. The U.K.’s biggest pub chain J.D. Wetherspoon said it was having problems getting kegs of John Smith's ale, made by Heineken, and Strongbow cider to some of its pubs.

“Heineken has been the company with the biggest issues and they have told us that all is getting back to normal,” a J.D. Wetherspoon spokesperson told NBC News’ U.K. partner ITV News on Wednesday.

Raquet said CO2 plants should be coming back over the weekend, but that it would take a few more days to get quality product to the necessary plants, meaning the shortage won't ease until the middle of next week.

In the meantime there’s still plenty of soccer to be played.

“We’ll have vodka shots,” German fan Hascher joked in the Bavarian Beerhouse.

“I’d just have a different drink instead,” her friend Jack Cheals, a 19-year-old student from Wales, said. “I don’t know. Just water?”