Peter Hain has announced he is resigning as Shadow Welsh Secretary.
The Neath MP says he's taken the decision to stand down to focus on new challenges - notably to lead a campaign in Parliament for the building of a Severn Barrage.
He told Ed Miliband of his intention to quit before last Christmas, saying he wants to make 'a different contribution'.
The 30 billion-pound Severn Barrage project would be the biggest renewable energy project in Europe.
Mr Hain says the campaign is 'the biggest contribution he can make' and that 'it's time to make it a reality'.
Severn Barrage wld be UK's biggest new investment project & create thousands of new jobs. This is biggest contribution I can make right nowFrom @PeterHain on Twitter:
Severn Barrage also biggest renewable energy project in Europe - time to make it a realityFrom @PeterHain on Twitter:
He says he also wants the freedom 'to contribute much more on wider policy issues', particularly Africa, 'where there are now both enormous opportunities for sustainable development and obstacles to be overcome.'
But Mr Hain says he'll remain a prominent voice in Welsh politics and will stand again as MP at the next election. He has served on Labour’s frontbench – both in Government and in Opposition – for around 16 years.
– Peter Hain MP
My intention is to re-stand as MP for Neath at the next election and to continue to fight hard and campaign hard for Labour across the country.
Meanwhile, Friends of the Earth Cymru have hit back at Hain's plans for the Severn barrage saying it has "the potential to be environmentally devastating."
The Director of the environment group, Gareth Clubb, said the tidal barrage would "destroy" vast areas of habitat and "devastate" fish and bird life.
The group says it agrees that using Wales' natural resources for renewable energy is important in tackling climate change, but believe that the barrage could be the "wrong solution."
– Gareth Clubb, the Director of Friends of the Earth Cymru
Tidal energy can be captured by other means with much less damaging consequences, and could be giving us green energy far sooner than the 20 years it will take to build this barrage.