In a statement this morning Tullett Prebon said it had completed its acquisition of the business under an all-share deal worth £1.28bn.
The deal will give Nex shareholders a 56 per cent stake in the new company.
Last month Tullett Prebon got the nod from the Financial Conduct Authority over the deal, which meant the Icap brand was transferred to Tullett and the new group was renamed Nex Group.
"This deal creates the leading interdealer broker in the world. It is a transformational acquisition for TP ICAP, providing a unique opportunity to accelerate our strategy and deliver for our clients, our employees and our shareholders," said TP Icap chief executive John Phizackerley.