A former senior Facebook executive has told how the social media giant worked "hand in glove" with the Chinese government on potential ways of allowing Beijing to censor and control content in China.

Sarah Wynn-Williams - a former global public policy director - says in return for gaining access to the Chinese market of hundreds of millions of users, Facebook's founder, Mark Zuckerberg, considered agreeing to hiding posts that were going viral, until they could be checked by the Chinese authorities.

Ms Williams - who makes the claims in a new book - has also filed a whistleblower complaint with the US markets regulator, the Securities and Exchange Commission (SEC), alleging Meta misled investors. The BBC has reviewed the complaint.

Facebook's parent company Meta, says Ms Wynn-Williams had her employment terminated in 2017 "for poor performance".

It is "no secret we were once interested" in operating services in China, it adds. "We ultimately opted not to go through with the ideas we'd explored."

Meta referred us to Mark Zuckerberg's comments from 2019, when he said: "We could never come to agreement on what it would take for us to operate there, and they [China] never let us in."

Facebook also used algorithms to spot when young teenagers were feeling vulnerable as part of research aimed at advertisers, Ms Wynn-Williams alleges.

A former New Zealand diplomat, she joined Facebook in 2011, and says she watched the company grow from "a front row seat".

Now she wants to show some of the "decision-making and moral compromises" that she says went on when she was there. It is a critical moment, she adds, as "many of the people I worked with… are going to be central" to the introduction of AI.

In her memoir, Careless People, Ms Wynn-Williams paints a picture of what she alleges working on Facebook's senior team was like.

Mr Zuckerberg, she says, did not get up before midday, loved karaoke and did not like to be beaten at board games, such as Risk. "I didn't realise that you were supposed to let him win. I was a little naive," she told us.

However, Ms Wynn-Williams says her allegations about the company's close relationship with China provide an insight into Facebook's decision-making at the time.

"China is Mark Zuckerberg's white whale," meaning a goal that he obsessively pursued, says Ms Wynn-Williams.

The country is the world's biggest social media market, but access to Facebook remains blocked there, alongside the likes of X and YouTube.

"It's the one piece on the board game that he hasn't conquered," she says.

Ms Wynn-Williams claims that in the mid-2010s, as part of its negotiations with the Chinese government, Facebook considered allowing it future access to Chinese citizens' user data.

"He was working hand in glove with the Chinese Communist Party, building a censorship tool… basically working to develop sort of the antithesis of many of the principles that underpin Facebook," she told the BBC.

Ms Wynn-Williams says governments frequently asked for explanations of how aspects of Facebook's software worked, but were told it was proprietary information.

"But when it came to the Chinese, the curtain was pulled back," she says.

"Engineers were brought out. They were walked through every aspect, and Facebook was making sure these Chinese officials were upskilled enough that they could not only learn about these products, but then test Facebook on the censorship version of these products that they were building."