LONDON – The UK’s white-collar crime prosecutor is preparing to open a formal bribery investigation into Glencore and its work with Israeli billionaire Dan Gertler and the leader of Democratic Republic of Congo (DRC), according to two people with knowledge of the matter.

Investigators at the Serious Fraud Office (SFO) plan to seek formal approval for a full probe into Glencore’s dealings in Congo, said the people, who declined to be identified because the matter isn’t public. Since Glencore is based in Switzerland, the prosecutor would have to show it has jurisdiction because the company’s shares are traded in London.

Congo government spokesperson Lambert Mende said Congo is "absolutely calm" about the matter.

"We are ready to take all the blows and we are ready to fight back," Mende said by phone Friday. Any final decision on whether to proceed with a formal probe will be up to a committee of SFO senior staff, including interim director, Mark Thompson.

Spokespeople for the SFO, Glencore and Gertler declined to comment.

Glencore shares declined 25.55 pence, or 6.4%, to 372.30 pence at 1:36 p.m. in London, after falling as much as 6.6%. The shares, the biggest decliners Friday on the UK FTSE index, have fallen 2.1% this year.

Gertler and his close friend, DRC President Joseph Kabila, have been implicated in previous British and American bribery investigations. The US imposed sanctions on Gertler in December, saying he’d used his friendship with Kabila to corruptly build his fortune. The SFO has been scrutinizing the men’s relationship with a Kazakh mining company for six years. While Congolese officials have described the sanctions as unjust, Gertler hasn’t commented on them.

US hedge fund manager Och-Ziff Capital Management, which funded some of Gertler’s operations in Congo, admitted in 2016 to having conspired to bribe Congolese officials with the help of an unidentified Israeli businessman. Gertler has denied any wrongdoing and hasn’t been charged. Glencore cut ties with him and bought out his stakes in their joint ventures shortly after the Och-Ziff settlement.

An SFO investigation would add to a growing list of legal challenges for Glencore. It’s sought to distance itself from Russian oligarch Oleg Deripaska after the U.S. imposed sanctions in April. Glencore is also fighting Gertler over royalties he says the company still owes after they parted ways. And Glencore’s state-owned partner in Congo is trying to dissolve a local operation, saying Glencore has overburdened it with debt. The Glencore unit says the debt situation is solvable and dissolving the business is premature.

The SFO is also at a critical juncture. Director David Green, who established a track record pursuing high-profile cases, left his post last month. While the interim director has said the SFO will pursue cases "with vigor" while he is in charge, UK Prime Minister Theresa May has supported plans to fold it into another agency.

Gertler and Glencore first invested in a Congolese mine together in 2007 and developed a close partnership over the years in the Mutanda and Katanga Mining copper and cobalt operations. In 2012, Glencore Chief Executive Officer Ivan Glasenberg said Gertler had been a “supportive” shareholder in Katanga Mining and that his involvement helped attract foreign investment to Congo.

Congolese mines are integral to Glencore’s projected growth, with Katanga Mining alone expected to account for a fifth of Glencore’s global copper production next year.

The SFO’s interest in Glencore stems from its six-year probe into whether Kazakh mining company ENRC paid off Congolese officials with Gertler’s help, the people said. All three have consistently denied any wrongdoing and no charges have been brought.