However he will not comment on suggestions he is being forced out by the Left faction, to make room for new talent.

It is understood a meeting of the faction put Mr Corbell in an almost unwinnable position in its list of candidates for the next election.

But he says he is confident he would have retained preselection, if he had stood for the election.

Mr Corbell, the Deputy Chief Minister, will leave politics at the next election, after spending most of his adult life in the Legislative Assembly.

He has announced his retirement ahead of the Labor Party drawing up its list of candidates for next election.

He says he was surprised at his low first preference vote at the last election but wanted to serve another term, to work on projects such as wind farms, restorative justice and the tram.

"[My resignation] is a matter that I've thought about at quite some length over the past six to 12 months and for me I feel that last week was exactly the right time to make it clear," he said.

He said he was sorry a letter to the editor in The Canberra Times suggested his resignation may be linked to ill health.

The letter said: "If Mr Corbell's decision to quit is based even in part on health reasons, he would do well to consider moving to the backbench.

"When he was demoted by Mr Stanhope, he admitted he had been not performing well because of depression. The ACT deserves a deputy chief minister who is at the top of his form."

Mr Corbell said: "I was very disappointed to see that – no criticism of the paper – but for me that letter really highlights why it is still so important for speak about mental health and it highlights why so many people are reluctant to talk about depression.

"With me, we're talking about something that occurred over a decade ago but people still attribute my decisions, when they consider them wrong or poor, to that – this is exactly what this letter was about.

"Is it any wonder that men in particular, but people in general, don't talk about mental health when you can still have that finger pointed at you a decade after you've spoken about it.

"For me it only just reinforces the importance of talking about it and being up front about it."

Three usually reliable sources say a recent meeting of the Left faction put Mr Corbell in an almost unwinnable position, on its informal ballot paper.

One source said it could be viewed as a way of letting him know it was time for new faces in the Assembly.

"They have politely tapped him on the shoulder and said we need renewal," the figure said.

"The Left put him down the order of candidates to a point where it would be very hard to win and that was their code for it's time to bow out gracefully."

Another source said Mr Corbell did not attend the meeting and was put in the last position on the faction's list of candidates.

"He came last in the preselection, for two reasons – he didn't turn up for the meeting and there is rising anger amongst Labor Party members, particularly those on the Left, over the use of a public-private partnership for the light rail."

Mr Corbell said he would not confirm or deny the claims about the Left meeting.

"I'm not going to speak about what goes on inside the party," he said.

"It's never been my policy to speak about internal party machinations so I'm not going to be speaking … I'm not going to confirm or deny."