Ed Miliband has outlined radical changes to the position of union's within the party after alleged vote-rigging tactics in Falkirk.
But Unite has made clear it rejects much of what has been proposed.
It said it would not accept any move to dilute its voting strength at the party's annual conference.
Conservative chairman Grant Shapps said that unions would "continue picking (the party) leader, policies and candidates".
Labour will hold a conference next March to vote on the proposed changes.
Mr Miliband wants millions of trade union members to join the party on its own right rather than just pay affiliate fees.
In a statement the union said: "Unite welcomes any measures which increase the involvement of individual trade unionists in the Labour Party. However, it cannot support any proposal that would lead to the collective voice of Unite being expressed solely through individual Unite members scattered across the constituency parties, nor can it accept any diminution of the trade union vote at either Regional or National Conferences nor of our delegation to Conference."
Unite leader Len McCluskey added: "Unite is committed to Ed Miliband's vision of creating a mass membership party that strengthens the voice of ordinary working people, while maintaining our collective voice."