The move follows a decision by the Alliance party to ensure candidates in next year's Assembly promise to vote in favour of same-sex marriage at Stormont.
An Alliance spokesman said: "This change will come up at the party council in September. We expect it to go through without opposition."
There's speculation the whip on party members will see the measure passed on a simple majority in the Assembly, though the DUP could still veto it by raising a petition of concern.
The DUP, which is Stormont's largest party, has previously not allowed its members a free vote, and has indicated that it is unlikely to do so in the future.
"Members of the assembly have voted against the introduction of same-sex marriage on four separate occasions," said the party's chief whip Peter Weir MLA.
"The DUP is opposed to the redefinition of marriage and it is a matter for other parties to explain their own stance on the issue," he added.
The Alliance party's stance appears to have provoked senior church representatives to call for all party whips to be removed on the controversial issue.
When same-sex marriage came to a vote in April, it was defeated by 49 to 47.
That included three Alliance abstentions, so if they were forced to vote yes and the parties had the same strength in the next Assembly, the vote would be 50 to 49 in favour.
Following the vote in May, Northern Ireland is now set to be the only part of the UK or Ireland where civil marriage is denied to same-sex couples.
Sinn Féin president Gerry Adams recently suggested it was "only a matter of time" before there is marriage equality across the island of Ireland.
