Heath Lambert and US broker Acordia are combining their retail networks, but are stopping short of a full scale merger.
The joint revenue of the two brokers will be more than £1bn, propelling them into the global top five.
Heath Lambert and Acordia plan to target retail firms around the world and offer them an enhanced multinational service.
A joint private holding will be set up for the venture, called HLA Global, in Guernsey in the Channel Islands.
The companies said they would not be pursuing a full-scale merger but there would be some rebranding and joint marketing.
“This venture is undertaken on the basis that we have two organisations with very similar philosophies, in that we both aim at medium-sized to large companies,” said a Heath Lambert spokesman.
Acordia will gain access to the Lambert Network, which was formed in 1998 to provide cross-border retail insurance services to clients, and now spans 54 countries. The move will also give Heath Lambert an extensive US servicing capability.
A spokesman for Acordia said: “HLA Global will satisfy the increasing international risk management needs of our clients with existing or expanding operations in global markets.
“We believe this retail partnership will give US-based multinational companies a responsive alternative to their risk management needs.”
HLA Global will be divided into regions in order to “harness the unique local insight of each member”.
Lloyd's broker Heath Lambert is the world's sixth largest insurance broking and risk management organisation and employs 4,000 staff.
Acordia is the largest privately-owned US insurance brokerage, with 3,800 employees in more than 100 local offices and its HQ in Chicago. It is the world's seventh largest insurance brokerage.