Former SDP leader Lord Owen has made a donation to Labour coffers and praised the "brave and bold" reforms of the party's links with trade unions pushed through by Ed Miliband.
The peer served in a number of high-profile government roles for Labour, including foreign secretary, before leading the breakaway "gang of four" over differences on Europe and defence after a bruising party conference in 1981.
After far-reaching changes to Labour's structure were overwhelmingly agreed at a special conference in London, Lord Owen revealed he has given up his crossbench status in the House of Lords to make a contribution of " over £7,500" to his former party, but will remain independent.
Lord Owen's decision to form a new party led to bitter recriminations for many years and the donation will be seen as highly symbolic.
The former Cabinet minister, who was behind a bid to delay the Health and Social Care Bill in 2012, said he hopes Labour will come up with a plan to "rescue" the National Health Service, which he claims will be "completely destroyed" without a change of government.
He said: " This is a brave and bold reform by Ed Miliband and one I strenuously argued for as a Labour MP at the special conference on Saturday, 25 January 1981.
"This very desirable change, nevertheless, threatens to weaken Labour's financial support at a critical time when I and many others are hoping to see the party produce a plan for government from May of next year to rescue our NHS.
"Saving the NHS is my main political priority and I suspect that of many others. To help Labour reverse the 2012 NHS legislation without yet another major reorganisation, I have made a declarable contribution of over £7,500 to Labour funds. Unless there is a change of government the NHS in England will be completely destroyed by 2020.
"I want to support Labour but also value my independence. I have informed the Convenor of the Crossbenchers in the House of Lords of this. I will continue to sit, as allowed under their rules, on the Crossbenches as I have done from 1992. However, since I can no longer be called a Crossbencher under their rules, I will now be an independent Social Democrat."
Mr Miliband revealed he has had many conversations with Lord Owen over recent years and values his "friendship and insight".
The Labour leader said yesterday: "Today Labour has come together and shown the courage needed to change our party. The reforms agreed today are supported across the party, by trade union general secretaries, grassroots activists and former prime ministers.
"In the 80s and 90s these reforms were seen as impossible but there is broad consensus within the Labour Party that change must happen. That is testament to how far we have come as a movement.
"Lord Owen's support today is welcome. It is 33 years since he left our party and much has happened since. In our many conversations over the past few years, I have come to value his friendship and insight into politics. I value his support and respect his decision to remain an independent member of the House of Lords."
Mr Miliband's reforms, devised following controversy over Unite's involvement in the selection of a Labour candidate in Falkirk last year, were backed by 86% to 14%.
The Labour leader insisted the biggest transfer of power in its history would help build a strong party that would help make the voices of working people "louder".
A defiant Len McCluskey said the changes would increase trade union involvement in the party and insisted Unite was "going nowhere".
Mr Miliband told unions, MPs and other delegates he had taken a "big risk" last July when he proposed the reforms.
"I did not believe we could face up to the challenges the country faced if we didn't face up to the challenges faced by our party."
Mr McCluskey told the conference the reforms would "start to take us down the road of involving more trade unionists in the business of the party" and insisted his union "did nothing wrong" in Falkirk.
He added: "Let me finally say to those elements inside the party who seek to edge us out, or to the grandees who snipe from the sidelines - this is our party and we are going nowhere."
Conservative Party chairman Grant Shapps said: "These reforms are a big victory for the unions, increasing their powerbase and ensuring they remain the dominant players in labour politics for years to come."