The 26-year-old returned an adverse analytical finding for trace amounts of the substance clomifine, a fertility drug, following a sample taken by the Voluntary Anti-Doping Association.
The B sample has yet to be tested and promoter Matchroom Boxing said in a statement the fight at the O2 would still go ahead.
In the statement, it said: “Mr Benn has not been charged with any rule violation, he is not suspended and he remains free to fight.
“Mr Benn has since passed a doping control test conducted by the UK Anti-Doping Agency, the anti-doping authority to which the British Board of Boxing Control has delegated its doping control testing for the bout. Mr Benn has passed all doping control tests conducted by UKAD.
“Both fighters have taken medical and legal advice, are aware of all relevant information, and wish to proceed with the bout this Saturday.”
Eubank Jr had labelled this weekend’s dynasty fight a trilogy in the wake of his father twice boxing Benn’s father, Nigel, three decades ago. Eubank Jr had dropped down to 157lbs for the meeting – his lowest weight since the age of 18 – while Benn has had to put on 10lbs to make the step up from his preferred weight.
Both fighters are expected a good six-figure sum for their clash in London, which looked to be in doubt when news emerged of Benn’s adverse analytical finding. But Matchroom were quick to quash any doubts about the fight, which is expected to be watched by 20,000 fans, going ahead.
Both fighters are expected in front of the media on Wednesday for a pre-bout workout session.