An advert for Huel meal replacement shakes has been banned by the advertising watchdog for encouraging or condoning "irresponsible" dietary practices.

The ad featured Huel brand ambassador Spencer Matthews sharing an anecdote about a person he met who "only eats Huel" and who won a 260km race "by miles".

The Advertising Standards Authority (ASA) said that viewers could assume from the ad that replacing all conventional food with Huel was "nutritionally appropriate" and was compatible with elite physical performance.

Huel said Matthews' anecdote was not "a recommendation or instruction" and the purpose of the ad was to challenge the misconception that Huel could replace every meal".

The video, which appeared as a sponsored Instagram post and a YouTube pre-roll ad, featured Matthews speaking with Huel founder Julian Hearn in a podcast-style format.

The Instagram post included the caption: "Here's how our founder uses Huel. Spoiler alert it's not for every meal."

In the video, Hearn said he has Huel products for breakfast and lunch during the workweek, but has a "traditional meal" when he gets home.

In response, Matthews said that while competing in an ultramarathon, he met a man who "only eats Huel".

Matthews said he was initially sceptical that the man would be able to manage the race, but then said: "He won it by miles, like literally just left us all in the dirt."

Hearn replied: "In an ideal world, you should have nutritionally complete whole food.

"Most people either don't do that or can't have the time to do that, and therefore that's where Huel fits in the gap."

Huel, which is based in Hertfordshire, was founded by Hearn in 2015. It is currently being acquired by the French conglomerate Danone.

The market for so-called complete nutrition products, aimed at time-poor, health-conscious consumers, is thought to be worth $5.9bn (£4.4bn).

The ASA received four complaints about the advert, which challenged whether the content was irresponsible. Two of the complainants also challenged whether the health claims in the ad breached the ASA's advertising code.

The ASA acknowledged in its ruling that the ad "did not explicitly recommend that consumers replace all conventional foods with Huel", but consumers could nonetheless interpret the anecdote as "an acceptable dietary practice".

"By presenting exclusive consumption of Huel in that context, the ads suggested that the complete replacement of conventional food with Huel was nutritionally appropriate and presented no barrier to achieving a high level of physical performance," the ruling said.

The ASA also upheld the complaint regarding the health claims as it ruled Matthews' anecdote about the ultramarathon athlete "implied a beneficial relationship between consuming Huel and the ability to sustain exceptionally high levels of physical and endurance performance".

In its response to the complaints, Huel said the advert "unambiguously positioned Huel as part of a balanced diet for occasions when preparing whole food was not practicable".

The company pointed to Matthews's reaction of "extreme scepticism and disbelief" to the idea of a person consuming only Huel.

"The anecdote was recounted as a remarkable and unexpected occurrence, not as a representative outcome or a recommendation or instruction to consumers to replicate.

"At no point in the ads did either speaker suggest that consumers should adopt a 100% Huel diet."

It is not the first time that Huel has landed in hot water with the ASA over the use of its products as a replacement for conventional food.

Additionally, some experts have questioned the effectiveness of replacing meals with nutritionally rich drinks.

Huel told BBC News that it had complied with the ASA ruling.

"We do not position Huel as a complete diet replacement or claim that it improves athletic performance," a spokesperson said.