Former West German Chancellor Helmut Schmidt, a personification of probity for many Germans, has admitted at the age of 96 that he had an extra-marital affair many decades ago.

Schmidt, who ruled during a turbulent era from 1974 to 1982, said in a new book that his wife Loki offered to step aside for his mistress, but he turned down that option.

His confession was a stunning relevation considering he often publicly professed his love for his wife, who was a popular botanist and author. She died in 2010 at 91 after 68 years of marriage.

"I had a relationship with another woman," Schmidt is quoted saying in Stern magazine in excerpts from his new book "Was ich noch sagen wollte" (What I still wanted to say). Schmidt did not reveal many details about the affair other than to say:

"It was in the late '60s or early '70s that Loki offered to separate over it," he said, adding he was flabbergasted by her suggestion they break apart so he could be with the other woman.

"There's no way I can leave you," he said he told his wife. "That's a completely absurd idea."

Schmidt said he stayed in contact with the other woman and attended her funeral two years ago.

Schmidt took power after his Social Democrat (SPD) ally Willy Brandt was forced to resign as chancellor when a close aide was unmasked as an East German spy.

While Germans tend not to be bothered by leaders having extra-marital affairs, there were worries that Brandt, whose private life fed the rumour mills, and his government could be vulnerable to blackmail.

Schmidt became a newspaper publisher and author who has enjoyed improbable popularity late in life, partly for his gruff and direct style. His refusal to obey rules against smoking in public buildings, even live on TV, helped him win cult status.

The former chancellor was a soldier in World War Two and won the Iron Cross for his service on both the eastern and western fronts.

In an opinion poll for Hoerzu magazine three years ago, Schmidt was voted Germany's number one role model by 61 percent of those surveyed. Chancellor Angela Merkel was 10th, behind a long list that included comedians and entertainers.

"It's possible people believe I'm a role model for Germans because I embody decency and industriousness," he told Hoerzu in 2012. "I've also been a straight-talker my whole life."