A Roman silver drinking vessel that depicts two sets of male lovers is one of the most prized jewels in the British Museum, singled out by director Neil MacGregor for his critically acclaimed History of the World in 100 Objects.

But on Wednesday, 15 years after the British Museum bought the Warren Cup for £1.8m, a highly respected German archaeologist suggested it could be a forgery.

At a public debate staged by King's College London, Prof Luca Giuliani challenged the museum's view that it dates from the 1st century AD.

The professor of classical archaeology at Humboldt University in Berlin dismissed it as a creation of the early 20th century, arguing that such explicit imagery is unprecedented in Roman silverware. He suggested instead that the cup was designed for the pleasure of its former owner – a wealthy American gay man, Edward Perry Warren, who bought it in Rome in 1911, and who also acquired other "counterfeit" pieces, he said.

At the debate, which was sponsored by the King's classics department and institute of classical studies, the British Museum's position was defended by Prof Dyfri Williams, author of The Warren Cup, published by the British Museum Press in 2006.

After Warren's death in 1928, the cup remained in private hands, too explicit to be shown in public for tastes at that time. It features male lovers in various poses. One pair shows the erastes – an older, active lover – who is bearded and wears a wreath, and the eromenos – the younger "beloved" – who is a beardless youth. Another scene features a beardless erastes and an eromenos who is just a boy.

Giuliani's doubts were aired in Germany last year, but Wednesday marked the first time he has addressed a British audience on the subject.

He acknowledges the high skill, but much of his doubt were based the fact the iconography suited Warren's specific taste – and the fact that this is supposedly a unique Roman item: "There is no other Roman silver tableware with a comparable subject matter. Silver vessels have a completely different iconography. Sexual escapades have no place here." Parallels are only found in lesser material – pottery – he argued.

Speaking to the Guardian just before the event, he said such highly explicit imagery is completely unknown from the Roman world: "You never find any such example." But it is comparable to pornographic imagery available in the 1900s, he said.

Williams paid tribute to his adversary, describing him as "a very intelligent, highly respected scholar, very important person in German scholarship". He told the Guardian: "I wouldn't want to attack him on a personal level at all."

But he disagrees with his theories. The fact that Warren bought other fakes is irrelevant, he said. He also dismissed the uniqueness of the iconography as not being proof: "We're really only reacting to each piece when it's found. We may find something spectacular next week."

He added: "The real issue, which he has not addressed, is the object itself … If the cup was made around 1900, as he claims, they would be using virtually pure silver. They have been refining silver since the middle of the 19th century."

Williams argued that the inner side of the cup had signs of ancient corrosion and showed an image, but Giuliani said he would want to see further evidence.

• This story was updated on 12 March 2014 and on 18 March 2014 to reflect the discussion at the debate.