Former Welsh Secretary Paul Murphy says he will vote in favour of more law-making power for the Welsh assembly in the referendum on 3 March.
He campaigned against devolution in the first referendum of 1979 and has previously struck a cautious tone on the assembly.
But he said increasing the assembly's power would help it "stand up" for Wales against UK government cuts.
True Wales, which wants a No vote, said devolution had a poor record.
Writing in the Western Mail, Mr Murphy said voting Yes was part of a "quest for social justice".
The Torfaen MP, who twice served in the Cabinet under Tony Blair and Gordon Brown, has described himself as a "devo-realist".
He said: "I will be voting Yes to support progressive policies and give us a stronger voice to stand up against the cuts agenda of the Tory-led government.
"Because more than ever, Wales needs the protection of the National Assembly against a UK government not acting in our best interest."
He said he was not a fan of "identity politics" and that talk of a "national project - some mythical quest for emancipation - has always struck me as nonsensical".
He said he remained committed to the Union and that Wales was better off as part of the UK.
"I hear the concerns about the 'slippery slope' to separation," he said. "But what I would say to anyone concerned about that is yes, stand up for the Union.
"Point out the strengths of Wales within Britain, challenge the separatists to explain how we would be better off if Wales were independent."
But he said the issue of independence was not part of the referendum, and that questions over devolving tax-raising powers and powers over policing and justice were "irrelevant distractions to the real issues facing Wales".
"What this vote is about is good governance and a stronger voice for Wales."
He added: "Having been lucky enough to be involved at the heart of Welsh governance for most of the first decade of devolution, I have seen the assembly grow in stature from an uncertain beginning.
"No-one is arguing for one second that all the assembly has done is perfect, but its influence on Wales and its people has been a positive one."
In a statement, True Wales spokeswoman Rachel Banner said Wales had received record budgets and billions from the EU, but politicians in the assembly had failed to close the "wealth gap" with the rest of the UK.
"It is, therefore, a bit rich for senior politicians in the Yes camp to claim that the assembly will shield us from UK coalition government cuts when they themselves left our country in this weakened economic position when we entered this grim era of recession and cuts," she said.
Meanwhile, a cross-party group of 11 councillors in Wrexham announced they would vote No vote on 3 March.
Liberal Democrat councillor Phil Wynn said: "I have come to the conclusion that if we want a prosperous Wales then we don't want a separatist agenda from the United Kingdom.
"I believe a Yes vote is a step nearer to independence.
"We are not saying No never, we are just saying not now and let's send a message to Cardiff Bay."