Labour leader Ed Miliband has won backing from his predecessor, Tony Blair, for radical reforms he is proposing to the party's relations with the unions.

The former prime minister hailed Mr Miliband for "real leadership" as he risked a clash with the unions by setting out plans to end the system where individual union members are automatically affiliated to the party.

In the wake of the Falkirk ballot-rigging scandal, the Labour leader will use a keynote speech today to set out what aides are describing as the "biggest party reforms in a generation".

The changes are intended to strengthen the party's links with its individual members while diluting the influence of the trade union barons.

But even before he delivered the speech, Len McCluskey, the general secretary of Unite - the union at the centre of the Falkirk row - indicated that he would oppose the plan.

He said: "Switching to an 'opt-in' for the political levy wouldn't work - it would require Labour to unite with the Tories to change the law, would debilitate unions' ability to speak for our members and would further undermine unions' status as voluntary, and self-governing, organisations."

Meanwhile, Conservatives released a second leaked document detailing Unite's involvement in the candidate selection process in 41 constituencies, which Tory chairman Grant Shapps claimed showed that "a cartel of union barons are running the Labour Party".

Welcoming Mr Miilband's move as "bold and strong", Mr Blair admitted that he should have made the same reforms himself when he was leader from 1994-2007.

"I think this is a defining moment, and I think it's bold and it's strong. It's real leadership," Mr Blair told Sky News.

"I think it's important not only in its own terms, because he's carrying through a process for reform in the Labour Party that's long overdue - and, frankly, probably I should have done it when I was leader - but at the same time, what he is doing - and I think this is also very important for the country - is that he's sending a very strong message to the country that in the end he'll do what's right for all the country and not simply for one section of it.

"This is big stuff and it takes a real act of leadership to do it."

In his address to the St Bride Foundation in London, Mr Miliband will call for an end to "the politics of the machine" - typified by events in Falkirk where Unite is accused of trying to pack the constituency with its members to secure selection of its preferred parliamentary candidate.

"What we saw in Falkirk is part of the death-throes of old politics. It is a symbol of what is wrong with politics. I want to build a better Labour Party - and build a better politics for Britain," he is expected to say.

Under the proposals, the three million trade unionists currently affiliated to the party through the automatic payment of affiliation fees will in future decide as individuals whether they wish to do so.

In other measures, Mr Miliband will announce plans for Labour's next candidate for mayor of London to be picked through a system of US-style primaries - with the possibility they could be extended to the selection of parliamentary candidates where the local constituency party is weak.

There will also be spending caps in selection contests for Parliament and the European Parliament covering both would-be candidates and any organisation backing them.

A new code of conduct will be drawn up for those seeking parliamentary selection, with the prospect of disqualification if they breach the rules.

Standard constituency agreements with the trade unions will be put in place intended to ensure that no one involved in the selection process can be subjected to "undue local pressure".

Officials acknowledged that ending automatic affiliation - which raises £8 million a year - would represent a financial "hit" for the party.

However Mr Miliband will argue that it will also provide the opportunity to mobilise trade unionists to get them to become active in the party, involving them directly in its campaigning.

"The problem is not that these ordinary working men and women dominate the Labour Party - the problem is that they are not members of local parties, they are not active in our campaigns," he is expected to say.

"Trade unions should have political funds for all kinds of campaigns and activities as they choose. But I do not want any individual to be paying money to the Labour Party in affiliation fees unless they have deliberately chosen to do so.

"So we need to set a new direction in our relationship with trade union members in which they choose to join Labour through the affiliation fee.

"I believe this idea has huge potential for our party and our politics. It could grow our membership from 200,000 to a far higher number, genuinely rooting us in the life of more people of our country."

Labour sources insisted that Mr Miliband had always intended to deliver party reform, although there was no attempt to deny that the timing of the announcement was linked to events in Falkirk.

Crucially, however, it was emphasised that the changes would need to be "carefully implemented in detail and over time".

Mr Miliband will not set out a timetable for reform but instead it is expected he will announce the appointment of a "senior party figure" to work through the process of putting it into practice.

Labour sources said they did not believe that it would require a change in party rules, although they suggested they could "formalise" the new arrangements with a vote at party conference.

"We would like to work with the unions and local parties to bring it about. We want to do it in a co-operative way but there are other ways in which you can do it," one source said.

Speaking to reporters as he arrived at Unite HQ this morning, Mr McCluskey said he would wait until after the speech to give his definitive response.

"We are going to listen to Ed's speech today and then I would be delighted to make a statement afterwards," he said. "It would be wrong for me to say anything until I actually listen to what he has got to say."

Billy Hayes, general secretary of the Communication Workers Union, said the plan would make it more difficult for working people to be heard.

Mr Hayes told BBC Radio 4's Today programme that opt-in was "a very old-fashioned idea", adding: "Let's just be clear, this is all about dog whistles. It's about signalling to people, you know, somehow, there's a problem with the relationship between trade unions...

"I don't think there is. I think there is a problem with some people in the party (who) have a great deal of difficulty dealing with ordinary working people."

Asked whether he could stop Mr Miliband pushing through the reforms, he replied: "Well, let's see. Let's just see what happens in the process."

Mr McCluskey and other union leaders were this morning at a scheduled meeting at the TUC headquarters in London, where Mr Miliband's proposals were not discussed, said a TUC spokesman.

Conservatives highlighted a briefing note on a meeting of Unite's executive council, which was published on the website of Unite Now, a grouping of activists within the union. Unite said that the document had been prepared by the grouping, and was not an official briefing note for Mr McCluskey.

The note said that Unite's political department was working to "maximise the possibilities of working class and progressive candidates in the Labour Party Parliamentary candidate selection process" and to encourage "more Unite members to join and become active in the Labour Party", particularly in constituencies with forthcoming selections.

Unite was "working with other unions to ensure general agreement around candidates in selections" and had reached broad agreement with the "big four" unions, it said. Unite was "hopeful" that candidates it backs will be shortlisted for most of the key seats in the 2014 European elections, adding that "with broad union backing that they shall be the prime targets for the top places".

Some 30 Unite activists had taken part in a training programme in February and a further 20 were expected to do so after Easter, aimed at developing them to become candidates for public office, the note said.

The note said that discussions were under way to ensure "a progressive manifesto and campaign" for next year's Euro-elections, focusing on opposition to austerity and xenophobia and support for a "Social Europe".

However, it suggested that Unite was concerned that the unions do not have as much influence as they would like on the policy review process leading up to the Labour manifesto for the 2015 general election .

"Jon Cruddas is directing the Policy Review, involving shadow cabinet groups, and with a number of meetings at Westminster," it said. "This is not developing as well as we might like - and we are seeking to get bigger trade union involvement in this."

Mr Shapps said: "This just shows that a cartel of union barons are running the Labour party and choosing candidates for Ed Miliband.

"The extent of their influence over the Labour party is quite shocking and terrible for democracy.

"Ed Miliband's unable to stand up for hard-working members of the public. The only people he'll stand up for are his union paymasters who bankroll the party."