The agency won the business in a pitch against other COI roster agencies including Precedent Communications.
Lt Col Mike Smith, defence public relations for the Army, said: "Grand Union exceeded the brief and demonstrated a strong understanding of our requirement."
The site, which is visited by the general public as well as potential recruits and serving soldiers, currently provides basic information and links to its other divisions and special interest websites related to the Army. It is managed by over 2,000 content editors around the globe.
The Army believes it needs to enhance its web presence in order to "keep pace with the needs and expectations of its audience". Grand Union intend to work on the site's layout and architecture and utilise rich media to achieve this.
Rob Forshaw, a partner at Grand Union, said: "The Army is undergoing a great deal of change. This presents a number of exciting challenges for the brand, both offline and online."
Grand Union was appointed to the design, development and delivery category of the COI digital roster when the 115-agency list was announced in June.
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