Tony Blair's decision to give Peter Hain the dual role of Welsh and Northern Ireland Secretary has been attacked by Plaid Cymru.
Tony Blair and Peter Hain in south Wales a week before the election
The Neath MP takes on the Northern Ireland role in addition to his Welsh job in the prime minister's post-election cabinet reshuffle.
Plaid said it was "unfair in principle and in practice" to people in both Wales and Northern Ireland.
But Mr Hain said he would still work for the "best possible deal" for Wales.
Mr Hain, 54, replaces another Welsh MP, Torfaen's Paul Murphy in the Northern Ireland office.
Mr Murphy, 56, who was Welsh secretary from 1999, had been Northern Ireland Secretary since October 2002.
Mr Murphy will leave the government to become the chairman of the parliamentary intelligence and security committee.
It is the second time Mr Hain has been given a double appointment in government. Mr Hain had been combining his Welsh secretary role with leader of the Commons.
He accused critics of his latest double role of "saying the same old rubbish".
"If you look at what has been delivered over the last couple of years - important reforming measure to support Wales - nobody has been able to say any single duty or responsibility or procedure I haven't carried out," he said.
Mr Hain said he would "continue to serve all parts of Wales" and continue his "close working relationship" with First Minister Rhodri Morgan and his cabinet.
The reshuffle means the leader of the Commons role goes to former Defence Secretary Geoff Hoon.
Helen Mary Jones, who speaks on the constitution for Plaid, said Mr Hain's new job was bad news for "proper representation of Wales in the UK cabinet."
Torfaen MP Paul Murphy had been in the cabinet for six years
"In the previous government, Peter Hain was part-time secretary of state for Wales, combined with his duties as leader of the Commons," she said.
"Now we have the unlikely and unfair combination of Northern Ireland secretary and secretary of state for Wales.
"This is unfair in principle and in practice, not just to the people of Wales, but also to the people of the north of Ireland.
"At a potentially troublesome point in the peace process - when Ulster unionist leader David Trimble has lost his seat at Westminster - the Northern Ireland secretary will need to be physically present there on many occasions."
The change has also been criticised by Ian Paisley's Democratic Unionist Party, which said the job of Northern Ireland secretary was being downgraded.
Despite historically low unemployment rates and more people in jobs in Wales than ever before; low interest rates; and rising prosperity, there is still a major job to do, taking the people of Wales with us," he said.
Mr Hain, who will arrive in Belfast on Monday, has begun work on how to achieve a lasting peace settlement in Northern Ireland.
He first came to prominence as a radical Young Liberal in the campaign against apartheid in South Africa, where he lived until he was 16 and his activist family fled to Britain.
He led the 1969/70 Stop the Seventy campaign to disrupt the South African cricket tour of the UK, and helped found the Anti-Nazi League in 1977, the same year he joined Labour.
He spent 15 years working as a political researcher for a trade union, entering the Commons at the 1991 Neath by-election.