Inmarsat vice-president of IR and corporate communications Chris McLaughlin confirmed the group had hired Finsbury but declined to comment further.
Around five firms are believed to have pitched for the business, including Brunswick and Citigate Dewe Rogerson.
Inmarsat used Financial Dynamics (FD) as its retained corporate and financial adviser until shortly after private equity houses Apax Partners and Permira bought the company in 2003.
FD said it would not have been approached to pitch because of a conflict over its relationship with PanAm Sat Holding Corp, whose flotation the agency worked on in March. McLaughlin denied this was a consideration.
Confirmation of Finsbury's appointment follows that of investment banks JP Morgan Cazenove, Morgan Stanley, Lehman Brothers and Merrill Lynch as joint bookrunners to the flotation, which Inmarsat was expected to confirm yesterday (Thursday).
Set up in 1976 under a UN mandate, Inmarsat was originally an intergovernmental organisation (IGO) run to provide communications through its satellites for 900 ships in the international maritime community.
Under international maritime law, any ship over 300 tonnes must carry Inmarsat navigational equipment.
In 1999 the company was the first IGO to become a private firm.
Inmarsat now supports links for phone, fax and data communications to more than 287,000 ship, vehicle, aircraft and other mobile users.
Last November Inmarsat launched its I-4 satellite, which will deliver a 3G-compatible broadband data service to mobile users.
Inmarsat's tenth satellite will allow 500k per second data transfer from a laptop-sized device located anywhere in the world.
The company hopes to use the proceeds of the flotation to clear shareholder debts and fund expansion for the business.