Old Mutual said in a statement on Tuesday that the claim is "without merit and we will resist accordingly".
The investment firm said it expects its "managed separation" of assets to be completed by the end of 2018.
The group's managed separation involves splitting the business into four parts including Old Mutual Wealth, Old Mutual Emerging Markets, South African lender Nedbank and US firm Old Mutual Asset Management. Old Mutual wants to break up the business because it believes regulatory changes make the company too complex to run in its current form.
Old Mutual is due to report its 2017 results on Thursday.
Shares fell 0.3% to 256.3p in morning trade.