Beer sales are down 40% since pubs reopened and it is threatening to push already-struggling pubs out of business.
Landlords fear drinkers have got used to supping canned ale at home during the coronavirus pandemic.
Despite restrictions being lifted, people are still not back in the habit of popping down their local for a hand-pulled pint.
As a result fewer pubs are willing to serve real ale because - as a live product – it has a shorter shelf life and if not drunk quickly it has to be poured down the drain.
Brewers say the decline in real ale is putting a massive strain on the pub industry.
The British Beer & Pub Association – aka BBPA – revealed 76 million fewer pints of cask have been sold in the nation's pubs since the pandemic.
From April to July this year 113 million pints of real ale were supped.
That compares with 189 million during the same period in 2019 before Covid hit, equating to a loss in revenue to pubs and brewers of £243 million in the last four months alone.
Before the pandemic cask sales had already been declining – from 2014 to 2019 they dropped by 17%.
A BBPA spokesman said: "Cask ale is without doubt key to British brewing. It is a form of brewing unique to the UK and has a rich history dating back to the Middle Ages.
"It is an unfiltered, unpasteurised ‘live’ product containing yeast that is served from a cask container where it’s gently matured by secondary fermentation.
"When conditioned and managed correctly, the yeast in the cask settles to the bottom, leaving a clear, often bright, full-flavoured and naturally carbonated beer to be served from a handpump.
"Because it is a live product cask ale can only be purchased and consumed at a pub. This means cask ale plays a vital role in the well-being and viability of pubs supporting the communities they serve.
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"Given the inextricable link between cask ale and pubs the plight of cask ale is a real concern for the nations locals and beer drinkers.
"It has been driven by pubs looking to stock less cask ale because of uncertainty around trading and restrictions."
The association has declared the next seven days Cask Ale Week in a bid to boost sales.
Chief executive Emma McClarkin said: "The plight of cask beer is a huge concern for our sector.
"Pubs are the home of cask beer so if sales of it are declining then it means the viability of our pubs are reducing too.
"We all missed a proper pint of cask ale in the pub during lockdowns. We cannot take cask beer for granted anymore.
"With the sector reopen once more it is vital we promote our pubs and the range they have on cask which they so expertly keep and serve.
"Doing so will help our brewers and pubs in their recovery and ensure this uniquely British style of beer can recover to the glory it deserves.''
Kevin Georgel, chief executive of St Austell Brewery which brews cask ales Tribute and Proper Job, said: "Not only did the pandemic force our great British pubs to shut, but it stopped us from being able to serve and enjoy cask beer.
"The impact of this has seen an accelerated decline of this quintessential British beer.
"The uncertainty of Covid, lockdowns and restrictions has meant less pubs serving cask because it is a live product and has shorter shelf life.
"Likewise it has meant the spontaneity of going to the pub for a pint – the core cask ale drinker occasion – has diminished.
"There has never been a better time for pub goers to support their local, by choosing to drink fresh cask beer, that’s been expertly brewed, stored and poured.''