Google on Monday fired the author of an internal memo defending the gender-gap in Silicon Valley tech jobs as a matter of biology, according to media reports.

The move, which was not officially confirmed by Google, splashed fuel on a burning controversy about whether “political correctness” at the company was stifling free speech.

Google told AFP that the company “can't comment on individual employee cases.”

In an email to employees, Google chief executive Sundar Pichai supported the right of employees to express themselves, saying that much of what was in the memo is fair to debate, according to a copy of the message obtained by AFP.

"However, portions of the memo violate our Code of Conduct and cross the line by advancing harmful gender stereotypes in our workplace," Pichai said in the email.

"To suggest a group of our colleagues have traits that make them less biologically suited to that work is offensive and not OK."

Pichai noted that the company code of conduct calls for 'Googlers' to do their utmost to create "a culture free of harassment, intimidation, bias and unlawful discrimination."

However, he defended the author's right to criticise Google training, workplace ideology, and whether programmes promoting workplace diversity are adequately open to all.

"The author had a right to express their views on those topics," Pichai said.

"We encourage an environment in which people can do this and it remains our policy to not take action against anyone for prompting these discussions."

Recode, Bloomberg and other media outlets on Monday reported that the author of the controversial internal document was fired.

The news became a hot topic at Twitter, with some calling the employee a martyr and railing against Google for punishing him for expressing a view that went against Silicon Valley's efforts to promote diversity at tech companies.

The leaked internal document that triggered the controversy claimed "biological causes" explained the lack of women in tech industry leadership roles.

The screed, dubbed "sexist" by US media, went viral, reviving the simmering debate over a culture of sexism and lack of diversity in tech sectors.

"I'm simply stating that the distribution of preferences and abilities of men and women differ in part due to biological causes and that these differences may explain why we don't see equal representation of women in tech and leadership," read the 3,000-word fulmination.

According to the author, natural aptitudes of men allow them to become better computer programmers, while women have more "openness directed towards feelings and aesthetics rather than ideas", meaning they "prefer jobs in social or artistic areas."

In response to the leaked memo, Danielle Brown, Google's new vice president of diversity, told employees in an email that "it's not a viewpoint that I or this company endorses, promotes or encourages."

"I found that it advanced incorrect assumptions about gender," she said.

The controversy comes as increasing numbers of women are going public with complaints of gender-based discrimination in Silicon Valley.

Currently some 69 per cent of Google's employees are men, according to the company's latest figures, a proportion that rises to 80 per cent when it comes to technology jobs.

In 2016 at Facebook just 27 per cent of senior executives were women. At Apple, around 30 per cent of total employees are women.