Google has fired the founder and co-head of its artificial intelligence ethics unit, claiming she violated the company's code of conduct.

In a statement, Google said an investigation found Margaret Mitchell had moved files outside the company.

The ethics unit has been under scrutiny since December, following the departure of another senior figure, Timnit Gebru.

Both women had campaigned for more diversity at Google and raised concerns about censorship within the company.

Ms Mitchell announced the news in a tweet, which read "I'm fired".

For the past five weeks, she had been locked out of Google's systems, including her emails and calendars.

Ms Mitchell was a fierce critic of Google's and had expressed concern about Dr Gebru's departure from the company.

Dr Gebru, a leading Artificial Intelligence ethics researcher, says she was fired late last year after sending an internal email that accused Google of "silencing marginalised voices". However, Google claims she left the company.

She co-authored a research paper which she says she was asked to retract. The paper had pinpointed flaws in AI language technology, including a system built by Google.

Following a meeting on the paper, she sent an email to an internal group called "Brain Women and Allies", criticising the decision.

Dr Gebru had emailed her management laying out some key conditions for removing her name from the paper, and if they were not met, she would "work on a last date" for her employment.

According to Dr Gebru, Google replied: "We respect your decision to leave Google... and we are accepting your resignation."

The fallout caused many within the scientific community to question the ethics of conducting research with big technology companies.

Hundreds of colleagues signed a letter in support of her. In response, the company's chief executive Sundar Pichai apologised for the way she left the firm.

Google has been criticised for a lack of diversity in its workforce, and there are concerns about the AI technology that underpins many of its services.