Transphobic abuse was aimed at a Pendle Labour member in toilets at the venue for the party's conference, it has been alleged.
Councillor Patricia Hannah-Wood said she had been abused by a fellow delegate as she spoke to the conference hall in Brighton on Tuesday.
Ms Hannah-Wood, a trans woman, told delegates: "On Sunday evening ... I was in the loo downstairs with a few of my colleagues from the North West when I was transphobically abused in this conference centre by one of our sisters in this conference hall now.
"It should not happen, we have got an anti-bigotry and anti-bullying policy in the Labour Party. We need it to be enforced."
She made the allegation as she moved a motion on LGBT+ rights.
It urged the party to recognise the "obstacles and growing violence" faced by LGBT+ people and to back moves to make so-called conversion therapy illegal and its use on under-18s a "form of child abuse, punishable by law".
The motion, supported by conference, also asserted trans people's "same right for self-determination as anyone else" and called for equal access to domestic abuse/rape support and shelters.
Ms Hannah-Wood said the LGBT+ community is "under constant attack", adding: "Within our own families, without our families, within our workplace, here within our conference. This is not acceptable."
Councillor Kevin Brady, of LGBT+ Labour, said the "fight for basic human rights" for LGBT+ people is "very much a live issue."
He told delegates: "The idea that progress is an unstoppable force never to be curtailed, the idea that we can't start to slip backwards is dangerous and untrue. Hate crime is rising in the UK."