Guinness sales have fallen after cheaper rival stouts undercut its dominance of British pubs.
Owner Diageo, which also makes Johnnie Walker and Smirnoff, said sales in the UK and Ireland had fallen by "low single-digits" between July and September because of "unusually hotter weather and competitive pressure".
Sir Dave Lewis, its chief executive , admitted that rival stouts were increasingly challenging Guinness in pubs and supermarkets.
He said: "Success draws competition. Guinness has been very successful. The competition is within stout.
"There's some work for us to do there. Not rocket science but we need to do it."
He also said the record-breaking heatwave this summer was a factor.
"We've had a very warm summer so the stout market does have some impact," he added.
The slowdown marks a surprise reversal for Guinness, which has enjoyed a boom as younger drinkers flock to the beer thanks to its popularity on social media.
A pub challenge known as "splitting the G", where drinkers use their first sip to try to get the head of the pint to align with the Guinness logo, has attracted increasing numbers of young people to the drink.
Rival brewers have sought to capitalise on the stout boom by tapping into the trend and offering cheaper alternatives.
Murphy's Irish Stout, made by Heineken, recently struck a deal with Wetherspoons to be stocked across nearly 800 pubs, charging an average of £2.99 a pint compared with £3.80 for Guinness.
Beamish, also owned by Heineken, recently expanded in the UK after striking a distribution deal with a major pub supplier.
Guinness copycats have also emerged from the big breweries including Caffrey's Black Stout, made by Molson Coors, as well as Tennent's Stout and Jeremy Clarkson's Hawkstone Black.
Diageo was forced to ration Guinness supplies to some British pubs two years ago. The stout's share of beer sales in British pubs almost doubled from 9.3pc in 2022 to 17.5pc in March 2025, according to company figures.
Guinness's dominance has historically given Diageo considerable pricing power, with publicans reluctant to stop stocking a drink that customers specifically demand.
However, landlords have repeatedly complained about its wholesale cost, which Diageo increased by another 5.2pc in April.
Cheaper rivals threaten that advantage by offering landlords potentially better margins.
Sir Dave, who took charge in January to revive the company's fortunes , suggested Diageo could make targeted adjustments to remain competitive, although he ruled out sweeping price cuts.
He said: "We're not talking about a dramatic repositioning here. We're talking about precision, portfolio-specific work."
However, he insisted Guinness had not lost market share in Britain or Ireland, arguing that the unusually warm weather had weakened demand for stout overall.
He also said there would be no repeat of the Guinness shortages that affected British pubs before Christmas two years ago.
Despite the slowdown, Guinness continued to record strong sales growth internationally, including in the US, Africa and parts of Europe.
Its American performance has been a rare bright spot for Diageo, whose spirits business has struggled amid weakening demand for premium drinks.
North American sales fell 3.6pc in the three months to September, with Don Julio and Casamigos tequila and Crown Royal whisky all recording double-digit declines.
Diageo's total sales rose 1.2pc to $4.93bn (£3.75bn), beating analysts' expectations.
Sir Dave is approaching his first anniversary in the job after launching a restructuring programme targeting $1bn in annual savings.
Diageo's workforce shrank by almost 2,000 in the year to June.
