The trade unions accounted for 77 per cent (£2.4m) of all donations to the party, with just £354,692 received in individual donations.
Labour figures believe the party fears the evidence against Unite is embarrassingly thin.
The block vote was abolished for Labour leadership elections 20 years ago. So why do David Cameron and the conservative press claim it still exists?
A survey by Lord Ashcroft shows 12% of the trade union's 1.42 million members would affiliate themselves to Labour under the new system but also that they oppose large donations to the party.
In an echo of Blair's revision of Clause IV, the Labour leader announces that a Special Conference will be held next spring to approve this "historic reform of Labour’s constitution".
With Labour more reliant on large one-off donations from unions, the Unite general secretary is in a stronger position to push for policy changes.
The Labour leader wants to keep his party united but he also wants to win an election. The two ambitions inevitably collide.
The reforms announced today will enhance Labour's traditional links and lay the foundations for new, open and powerful alliances.
The Labour leader shifts the focus from the unions by calling for new limits on MPs' outside earnings and a cap on party donations.
Labour suggests it will not impose the reform by changing the law with the Tories, but if the unions resist that may be the only option.