The 157lbs catchweight fight at the O2 Arena was called off after it was revealed that last month Benn tested positive for trace amounts of fertility drug clomiphene, which elevates testosterone levels in men.
Eubank Jr reacted angrily upon hearing the news, saying on Twitter that "Conor got caught using an illegal substance so the fight is off. Can't believe it. He has escaped his schooling… for now."
The 33-year-old has followed that up with two more digs at Benn, who insists he is a "clean athlete".
"Just got back to my hotel room to be greeted by another random drugs test… which as always I'm happy to take. 10 years & never failed one," Eubank Jr said.
In another escalation in his war of words, he posted a video of Benn criticising Jarrell Miller after his 2019 bout against Anthony Joshua was cancelled because the American had tested positive for multiple prohibited substances.
"The biggest night of your life, the biggest fight of your career and you get tested positive for a banned substance….it baffles me," Benn said in an interview with Sky Sports.
"I'm not even at world level and my nutritionist has a go at me for taking vitamin C tablets if they haven't been tested. So the team he's got round him should say 'listen, check all of your substances, you're earning a few good quid here. Why put that at risk?'."
Eubank accompanied the video with the remark, "I mean I really just don't have any words at this point".
The bout was intended to be a continuation of the Benn v Eubank feud that saw the rivals' fathers battle it out in two ferocious contests in the early 1990s.
A catchweight of 157lbs was agreed for the sons to continue the family feud into a trilogy fight, meaning Benn would have to move up two weight divisions and Eubank lose three pounds.
Benn's father Nigel, who now lives in Australia, has been present throughout the build-up and on Friday morning he defended his son, who was on the cusp of a world title challenge.
"Hi guys, as you know we're in total shock. I've been with my son for the last 10 weeks and the training has just gone absolutely the best it could ever be," Benn said on Instagram.
"He's a dedicated trainer. He leaves no stone unturned and we'll get to the bottom of this.
"I love my son. I know he don't cut no corners, he just goes straight at it 100 per cent. But we'll keep you informed. We'll let you know what the next step is. But you know, he's a faithful trainer and he's a clean athlete. Speak to you soon. God Bless."