The executive of Unite held a special meeting to discuss a paper drawn up for Labour leader Ed Miliband by Lord Collins, a former union and party official.
Mr Miliband called for changes in the wake of a row over Unite's involvement in the selection of a Labour candidate in Falkirk.
He wants union members to opt in to join Labour rather than be automatically affiliated.
Unite will now support the proposals when they are discussed at a special Labour conference next month.
Labour's National Executive Committee earlier backed the proposals.
If approved, these reforms will give members a choice over whether they opt into paying affiliation fees to the Labour Party, together with an opportunity to become affiliated members of the party with the right to vote in leadership elections.
The "electoral college" for leadership votes would be scrapped in favour of a one-member one-vote system.