Leinster supporters say they were not allowed to distribute flags to travelling fans in the ground before Saturday's Heineken Champions Cup clash at Bath.
Leinster Supporters, the Irish province's official fans' group, apologised on its Twitter account, describing it as "disappointing news."
"We brought 1500 flags over today's game and @bathrugby are refusing to let us distribute them. They are saying that the flags failed a health and safety test," Leinster Supporters tweeted.
"These flags have been passed as safe by airports and stadia all over the world. We would like to apologise to supporters for this disappointing news. We did everything we could to get the flags from Bath, but they wouldn't release them."
Seats at the Recreation Ground were adorned with Bath flags, but the Gallagher Premiership club said Leinster's flags failed safety criteria.
Bath tweeted: "We're really sorry - the flags were tested against three safety criteria and failed all three, including fire.
"We have to take safety seriously, but we realise it's disappointing. We're really looking forward to the game & we know you'll bring the noise to the Rec today."
