Alun Michael, former First Secretary of Wales, has announced he is to stand down from the Welsh Assembly on May 1.
Mr Michael, who was widely seen as Tony Blair's man in Wales, resigned as First Secretary last month before losing a vote of no confidence called over the issue of European aid.
An MP and a Welsh Assembly member, Mr Michael said today he could best put his "energy to use" by carrying on at Westminster.
Labour MPs who are also Assembly members have been told by the Party that they must chose between the jobs.
Mr Michael said he had thought "long and hard" about staying on at the Assembly. But in a comment obviously aimed at former Welsh Secretary Ron Davies, he said a former leader on the back benches could have a "negative and destructive influence" on such a new institution.
Mr Michael, who is currently MP for Cardiff South and Penarth and an Assembly member for Mid and West Wales, said he had "bags of energy left" and wanted to stay in politics for many years to come.
His constituency Labour Party recently unanimously asked him to stay on as its MP, a request Mr Michael said he was "deeply touched by".
He said he would be a "passionate advocate" for devolution at Westminster and would do everything he could to ensure devolution was a success.
Mr Michael said he had no regrets about taking the job of Welsh Secretary and the ensuing job as the first leader of the Welsh Assembly.
But he said he would have liked to have stayed in the job for longer.
Mr Michael enjoys a successful career at Westminster. He was elected as Jim Callaghan's successor in the Cardiff seat in 1987 and immediately became an opposition whip.
He was promoted to the job of Labour's opposition spokesman on Welsh affairs before becoming Tony Blair's junior as opposition spokesman on home affairs.
He became a Home Office minister in 1997 working as Jack Straw's deputy. He took over from Ron Davies as Secretary of State for Wales in 1998 and stood down last July to concentrate on his job as First Secretary.
Because of the electoral system used for Welsh Assembly elections there will not be a by-election for Mr Michael's seat. Instead the second candidate on Labour's list for the seat, Delyth Evans, will take over.
Ms Evans, a former journalist, was a special adviser to Mr Michael when he was First Secretary.Reuse content