Derhalli is claiming £10m from his former employer Lehman Brothers at the High Court.
Bell Yard founder Melanie Riley, co-founder Richard Elsen and their former Cicero colleague Stephen Lock are handling Derhalli's PR in the case, which is expected to last three weeks.
The case revolves around how much Derhalli should have received from Lehman Brothers for his work in reclaiming debts owed to the bank in Russia after the 1998 crash in the value of the rouble.
Derhalli's lawyers claim the bank's emerging markets team, under Kerim Derhalli's management, was extremely profitable and that Derhalli successfully recovered millions of dollars.
They say Lehman had agreed a contract with Derhalli, under which he is owed £10m in unpaid pay and entitlements.
Lehman denies there was any legally binding contract and says Derhalli was properly paid for the work.
Derhalli has now moved to the post of global head of commodities for Deutsche Bank in London.