Sirius has poached the Davis Group in a tug of war with software giant Misys.
The software manufacturer won the Davis Group contract from Misys after a detailed investigation of the products available on the market.
Davis Group managing director Kevin Hancock said: "Once our decision was made to replace our current systems we established a clearly-defined set of objectives for the new software.
"By investing in the right technology we will be able to streamline and automate our operations further."
Though disappointed at the loss, Misys hit back by citing three customers that have recently moved the other way.
A company spokesman said: "In a competitive market where businesses continually review their IT systems there will inevitably be business wins and losses.
"As a market leader, Misys has and continues to win new customers from a variety of suppliers, for example, Merrick Hill Insurance recently joined us from Sirius, as have Academy Associates."
The defection of Davis comes at a critical time for Misys which is about to embark on a £250,000 roadshow to promote OASys and other products.