A BBC investigation of 52 lettings agents has found that two thirds were levying charges on tenants, but it is uncertain whether they are acting legally or illegally.
The agents - all in Belfast - were probed by BBC Northern Ireland and were levying fees of £15 to £60 on applications to rent a property. The agents say these cover administration costs including credit history checks and employment references.
Of the 52 letting agents, 35 told the BBC they were charging and 17 explained they were not, to fall in line with the ban that was introduced in England.
However, while a ban on lettings fees in England came into force a month ago today and while landlords and agents in Wales will be banned from charging extra fees from September, there is no such legislation in the north of Ireland.
In Scotland, letting agent fees were banned in 1984, and in 2001 in the Republic of Ireland.
The BBC says that with no devolved assembly sitting in Northern Ireland - because of a long-running dispute between political parties - no new specific legislation has been introduced in relation to the charges.
However, there was a judgement in 2017 which suggested tenants could not be charged upfront costs in Northern Ireland; this was based on the Commission on Disposal of Land Order 1986, applying specifically to Northern Ireland.
The campaigning charity Shelter, which has made clear its distaste for letting agents in the past, says the fees now being levied by Belfast agents are therefore illegal.
A Shelter NI tweet on Friday said: “These fees are ILLEGAL and we must push letting agents to stop this unlawful practice which places an additional financial burden on those who can afford it the least.”
Ellie Evans, of the housing charity Shelter NI, told the BBC that letting agents "know they can get away with it" and tenants "are too scared of retaliation".
She continued: ”[Tenants] don’t have the same structural power that estate agents have. If they say, this fee is illegal I don't want to pay it, [agents] might say that you might not get the property. Or people might still be in the property and don't want to ask for their money back in case they get evicted. It's just about power and, right now, the power is in favour of the letting agents."