At a special conference in London, the Labour leader will seek and is expected to succeed in changing the historic link between the party and the trade unions, sparked by the row last year over Unite's alleged involvement in the selection of a Labour candidate in Falkirk.
The reforms will include changing the auto-enrolment of trade unionists to the party to one of voluntary affiliation, as well as a move to one member, one vote for electing the leader.
Most unions will support the reforms but the changes will hit the number of affiliated union members and could, in the short term, see Labour's finances lose millions of pounds.
In a message of support last night, former leader Tony Blair praised Mr Miliband for showing real courage and leadership. "It is a long overdue reform that, as I said before, was something I should have done. It puts individual people in touch with the party and is a great way of showing how Labour can reconnect with the people of Britain," said Mr Blair.
Mr Miliband will tell his audience of MPs, trade unionists and party delegates that more and more people are turned off from politics and are becoming disengaged.
He will say there are thousands of ordinary working affiliated to Labour but there is no way at present of reaching them, adding: "I don't want to break the link with working people. I want to hear the voices of working people louder than ever before."
Mr Miliband will tell delegates: "Today, if you vote for these reforms you will be voting for Labour to be a movement again ... That's why we have to bring people in. Today, let's vote to change our Party. Let's build a movement. So that tomorrow, we can change our country."
But as he makes his pitch to the party to embrace reform, senior figures have expressed concern about his strategy, warning Labour is "behind on the economy and behind on leadership" in its battle with the Conservatives.
One MP said: "We should just acknowledge he killed his brother; that view of him has not gone away and it is not going to go away now, not in time for the next General Election. We should be upfront and try to tackle it head on."