Former deputy prime minister Sir Michael Cullen has been diagnosed with lung cancer.

In a statement on Friday evening, Cullen, who is 75, said he would resign from his roles as chairman of the Bay of Plenty and Lakes district health boards, effective Sunday.

"I took on these roles knowing that they would involve a heavy workload but offered a real opportunity to make a difference in our health system in the Bay of Plenty," he wrote in the statement.

"Ironically, it is totally unexpected news about my own health which has led to this decision," Cullen said.

HAGEN HOPKINS/GETTY Sir Michael Cullen is 75.

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"What was initially a CT scan of my heart has resulted in a clear diagnosis of Stage IV small cell lung cancer with multiple secondaries in my liver," Cullen said.

"Chemotherapy is likely to extend my lifespan somewhat, but it is clear to me I will not be in a fit state to carry on all that I have been doing in recent months."

Cullen said he'd already stood down "reluctantly" from his position as lead claims negotiator for Te Kotahitanga o Ngāti Tūwharetoa.

The only major role he would maintain in the meantime was as chair of the Earthquake Commission, he said, to see it through the release and the response to Dame Sylvia Cartwright's report into EQC and the Christchurch Earthquake sequence.

ROSA WOODS/STUFF Sir Michael Cullen, pictured in 2019. The former deputy leader of Labour has been diagnosed with cancer.

"I expect to make a decision shortly about the timing of my departure from that position," he wrote.

The Bay of Plenty and Lakes DHBs in a joint statement said they expressed their sadness at the news.

Cullen served as deputy prime minister from 2002-2008 under Helen Clark's Labour government.

He was also deputy leader of the Labour Party from 1996-2008. He retired from Parliament in 2009.

He also served as Minister of Finance, Minister of Tertiary Education, and Attorney-General.