AXA has agreed a delegated authority deal with PBS Holdings for nearly £20m in commercial capacity.
It has also agreed an undisclosed amount of personal lines capacity for UK underwriting.
PBS Holdings chief executive Jonathan Davey said all risks would be written on a joint capacity basis by AXA and Primary Insurance.
"There will be no selection of risks. The two companies will share every risk in fixed proportions."
The commercial capacity will form a book of property-based risks. Davey said these would mainly be commercial combined risks, with some capacity earmarked for package and agricultural property risks.
"They are all material damage based risks," he said. The commercial capacity will be divided between PBS Holdings, Primary Broker Services, ATD General, Europea and Rural Insurance.
Davey said the deal was a testament to PBS Holdings' experience of underwriting on behalf of its own insurance company.