Ed Miliband has welcomed the backing of former SDP leader and Labour foreign secretary Lord Owen for the reforms.
He has given up his position in the Lords as a cross-bencher after donating to Labour coffers following the "brave and bold" reforms of the party's links with unions.
The Labour leader said he enjoyed many conversations with Lord Owen and values his "friendship and insight".
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He said: "Labour has come together and shown the courage needed to change our party. Lord Owen and the other members of the 'gang of four' - former Labour ministers Shirley Williams, Roy Jenkins and Bill Rodgers - rocked British politics when they walked out on the party over differences on Europe and defence after a party conference in 1981.
The SDP scored spectacular by-election successes and later joined forces with the Liberal Party at the 1987 election before merging to form the Liberal Democrats.