Drinks company Thatchers has launched a bid to overturn a High Court ruling that denied its claims Aldi "copycatted" its cider.
The Somerset-based cider maker sued the German discount chain for allegedly infringing the trademark of its cloudy lemon cider.
Judge Melissa Clarke concluded there was a low degree of similarity between the products and no likelihood of confusion for consumers.
To help form her judgment she conducted a blind test of the two products and concluded they were different, despite making it clear she was "no expert" and "found the taste of the two products to be very similar".
On Tuesday, Thatchers went to the Court of Appeal to challenge the ruling.
Martin Howe KC, for Thatchers, said in written submissions: "This a case where the Court of Appeal can justifiably interfere with the judge's findings at first instance, and should do so.
"Her judgment contains conclusions that are not rationally supportable and her overall finding that there was no unfair advantage is predicated on errors of principle.
"Accordingly, and notwithstanding the high threshold for interference, Thatchers respectfully say this is a case where such interference is both appropriate and necessary, and request therefore that its appeal be allowed."
Mr Howe said there was no explanation in the judge's reasoning as to "why it was not to be expected that Aldi would gain an advantage as a result of the link she had held would arise in the minds of consumers".
Read more from Sky News:
Sara Sharif's father and stepmother jailed for her murder
Inside the drug factories that bankrolled Assad's narco state
Illegal casinos using video game to draw children into gambling
Michael Edenborough KC, for Aldi, told Lord Justice Arnold, Lord Justice Phillips and Lady Justice Falk it was not up to the court to interfere with findings of fact.
He said: "We say that [the judge] reached the right conclusions based on the evidence that was before her."
Mr Edenborough added: "They [Aldi] try to act within the law at all times.
"The way in which this has manifested itself is that there was no intention to benefit from the goodwill or the reputation."
He said Aldi does not "have an intention to do anything nefarious".
The hearing is set to conclude on Wednesday and a decision is expected in writing at a later date.