Prime Minister Julia Gillard's frontbench team has been dealt a major blow, with Defence Minister John Faulkner revealing that he will move to the backbench after the forthcoming federal election.

The veteran New South Wales senator has served as a minister in the Keating, Rudd and Gillard Labor governments.

He brought stability to the Defence portfolio after Joel Fitzgibbon became the first Rudd Government minister to resign for breaching the ministerial code of conduct.

The 56-year-old has been a central figure in election campaigns and leadership changes - including the change from Kevin Rudd to Julia Gillard.

He says it is inevitable some will suggest his stepping down is related to that leadership spill, but says "any such speculation is just plain wrong."

"Julia Gillard has my absolute support. No-one will work harder than me for the Government's re-election under Julia Gillard."

Senator Faulkner says he discussed his resignation with Ms Gillard last week and again two days ago, when she persuaded him to recontest his Senate seat.

"When Labor was returned to office three years ago, I made a commitment to Kevin [Rudd] and Julia [Gillard] to serve in the cabinet for the first term," he said.

"I reiterated this when I became Minister for Defence.

"I've been in full time politics for 30 years, including 21 years as a senator. I know this is the time to step down.

"I believe the government is well placed to win a second term, but I also believe in generational change."

Senator Faulkner has been a key election campaign aide for several former Labor leaders and has confirmed he will play a similar role this time, travelling with Julia Gillard.

He praised her predecessor Mr Rudd for running an "extraordinarily disciplined and effective campaign" in 2007.

His decision to step down follows that of another of Labor's elder statesmen, Lindsay Tanner, who will not be recontesting the seat of Melbourne at the coming federal election.

Today Ms Gillard said she believed Senator Faulkner had "a further contribution to make as a member of the parliament and member of the team".

Greens Leader Bob Brown has paid tribute to Senator Faulkner, saying he has been a responsible and honest Defence Minister.

"We can't ignore the very responsible and honest role he's taken as Minister for Defence," he said.

"I think we've been lucky to have him as Minister for Defence in the last several months and I think it's a great pity he's not going to be there in that role in the next Parliament."

Senator Brown says Senator Faulkner has shown a sense of humanity in his role.

"He has a humanity which I have to say I find missing from this Gillard Government," he said.

"He has a great conscience, he's always been an enormous contributor to a more fair and open and transparent process in Parliament."