Neil Kinnock would campaign against more power for the Welsh assembly in another devolution referendum.
Kinnock knows the further powers issue could prove divisive
The outgoing European commissioner said it was his duty as "a responsible Welshman" to oppose more powers.
"I think the addition of legislative or tax-varying powers at this juncture would work against Wales," said the former Labour leader and Islwyn MP.
He was a leading opponent of devolution before the 1979 referendum, when Wales voted against an assembly.
He told BBC Wales' Dragon's Eye programme he believed any referendum to extend powers in the immediate future would be "potentially seriously disadvantageous".
Mr Kinnock would be prepared for certain short-term divisions among the Labour party on the issue.
"I wouldn't want to divide the Labour party - patently I spent a lot of time putting it back together, " he said.
However he added: "Maybe one of the best ways of measuring the seriousness with which I approach this issue is to say - why should Kinnock, who is the original demander of unity, be prepared to have at least short-term schism, because of this argument about the next proposed stage of devolution?
"And the answer to that is - this is for keeps. This really does have a massive effect on the potential, the security, the opportunity of Wales.
"And I can't arrive at a judgement, frankly, which tells me that the addition of legislative powers or tax-varying powers is a means of resolving any of the problems facing Wales."
He said it was a "fundamental issue" for the part of the UK "and therefore the world" that was most important to him.
"Not only because it's my homeland, but because I've represented my constituency for 25 years - a long time.
"I owe Wales and my background in the Labour movement everything that I've got.
"If a person like me can't put something back in - at least in the form of their judgement and energy - then it's come to a pretty pass," he said.
"That you could have all these fantastic advantages and privileges given to you by your family, your community, your school, university, political party, trade union movement - the whole thing - and then to say that the time has come in my life where I just let things flow past."
If the former commissioner perceived the issue as being "neutral" he would be able to keep silent about it.
But he added: "When it actually will tell against Wales in my view, then you can't really shut up in those circumstances.
"I think that as someone who has been given the privilege of a platform, for most of my adult life, I don't think I've got a right to shut up in those circumstances."