British energy giant BP said Tuesday that its chief executive Bernard Looney has resigned "with immediate effect", after admitting that he had not been "fully transparent" about historical relationships with colleagues.

"Bernard Looney has notified the company that he has resigned as chief executive officer with immediate effect," the company said in a statement, adding that finance chief Murray Auchincloss would act as interim CEO.

Looney, 53, is leaving the energy firm after less than four years in the role.

BP said that in May last year, its board received and reviewed allegations from an anonymous source relating to Looney's conduct "in respect of personal relationships with company colleagues".

Looney disclosed "a small number of historical relationships with colleagues prior to becoming CEO" during the review, it added while noting that no breach of the company's code of conduct was found.

However, the board was given his assurances over his disclosures of past personal relationships, as well as his future behavior, according to BP.

"Further allegations of a similar nature were received recently, and the company immediately began investigating with the support of external legal counsel," it said, adding that the process was continuing.

"Mr Looney has today informed the company that he now accepts that he was not fully transparent in his previous disclosures," BP added.

"He did not provide details of all relationships and accepts he was obligated to make more complete disclosure."

BP said the board expects all staff to behave in accordance with the company's values.

"All leaders in particular are expected to act as role models and to exercise good judgment in a way that earns the trust of others," the company said.

Looney has been at BP since joining the British energy behemoth in 1991 and was appointed chief executive in 2020.

The company said no decisions had yet been made regarding any remuneration payments.

Looney had vowed to reposition BP as a leader in clean energy technologies, and gradually cut oil and gas production to reduce carbon emissions, but environmentalists have criticized the firm for inaction in that regard during his tenure.

Like many of its rivals, BP earlier this year unveiled record annual profits for 2022, thanks to soaring oil and gas prices following Russia's invasion of Ukraine.

However, earnings have fallen somewhat from those heights so far this year, as energy prices retreated for much of 2023.

Nonetheless, BP said in August it was hiking its dividend and returning $1.5 billion to shareholders by repurchasing stock.