AI pitched against four unnamed agencies to win the account, which will see it work with the Post Office's e-commerce team to develop its website into a key revenue channel.
Allison Wightman, Post Office e-commerce commercial manager, said: "We needed an agency that would share our vision and could work as a partner with us to create a richer and more rewarding online experience for our customers."
AI will also provide planning, technical and accessibility consultation -- said to be high on the Post Office's digital agenda.
Steve Eaton, Post Office e-commerce operations manager, said the agency would complement the skills of its in-house team.
"This will enable us to progress our strategic plans more quickly from business case through to roll-out," Eaton said.
Susanna Hurley, client service director of AI, said: "The Post Office's energy and enthusiasm is infectious and we are delighted to be a part of a very important stage of its business development. It will be one of the champions of the interactive industry in 2005."
Arnold Interactive changed its name to AI in London after it was acquired by AIM-listed marketing services company Media Square from Havas for £600,000. The deal saw it leave the Arnold network and lost the right to use the Arnold name.
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