Wales's Labour leaders have attacked Plaid Cymru, their coalition partners in the assembly government, as Labour's annual conference begins.
Mr Hain said Welsh Labour was proud to be behind devolution
Welsh Secretary Peter Hain and First Minister Rhodri Morgan said Plaid's new role has not "dented their separatist aspirations".
The accusation, in a party pamphlet, comes only days after the two parties signed a historic coalition document.
A Plaid MP said Labour was "both the party of devolution and of the Union".
At the Labour conference in Bournemouth on Sunday, Mr Hain and Mr Morgan will launch a pamphlet called a 'Partnership for Progress'.
The document talks of devolution and Wales' place within the United Kingdom.
It claims that Labour is 'Britain's leading pro-devolution and pro-Union Party'.
It attacks the Tories' for their "commitment to 'English votes for English laws".
They also attack Plaid Cymru, saying that their "new role in the Welsh Assembly Government has not at all dented their separatist aspirations".
Mr Hain told BBC Radio Wales: "We're proud to be a party that has been behind devolution...but devolution within the UK.
"We're both devolutionists and a party of the Union."
Full national status
He said Labour published the pamphlet because they "felt the need to define Welsh Labour as being proud of Wales, patriotic about Wales, but firmly of the belief it's in Wales' interest" to remain within the Union.
"We're stronger within the union of the UK but with devolution, giving us our own identity and our own decision making and our own voice," Mr Hain added.
However Plaid Cymru MP for Caernarfon, Hywel Williams, called on Mr Morgan and his cabinet members to distance themselves from "a group of self-serving Labour MPs who are only concerned about losing their jobs".
Morgan and Jones signed the One Wales document in the Senedd
"These Labour MPs claim that the current model of devolution represents the aspirations of the Welsh people.
"Plaid's new role in the One Wales Government will certainly not dent its aspiration to secure full national status within the European Union for our nation.
"Only this week a poll confirmed that 83% of the people of Wales now support self-government - with a clear majority of the Welsh electorate supporting a full law-making and a tax-varying Parliament for Wales," Mr Williams said.
Mr Morgan and Ieuan Wyn Jones, deputy first minister, signed the One Wales document in the Senedd in Cardiff Bay.
The deal was sealed following wrangling after May's assembly election.