Esure called the review of its digital media business, which had been held by Equimedia since 2008, in August 2011. Carat holds the offline business, which was unaffected.
It is understood that Equimedia re-pitched for the account and that the other agencies involved were Steak and LBI.
The Aegis agencies will be responsible for driving awareness and increased customer growth for both insurance brands across affiliate marketing, social media, analytics, paid and natural search as well as conversation optimisation.
IProspect will help to optimise esure's search performance across its PPC, affiliate and display channels and deliver enhanced data management and attribution modelling.
Carat has managed esure's offline planning and buying since 2004.
Dorothy Fulton, head of direct acquisition at esure, said iProspect had impressed with their understanding of the brand.
She said: "Their industry insight will help us achieve our results by adding an edge to our overall marketing mix throughout the digital marketing space."
Ben Wood, managing director of iProspect, said: "This win shows the depth of iProspect's integrated proposition.
"Data, insight and analysis are at the heart of our business and we are looking forward to drawing on all of our resources to delivering impressive results for esure in the years ahead."