Proximity London will begin working on the HP reseller campaign worth $6m (£3.4m) annually immediately.
The agency will develop integrated marketing campaigns for each of HP's reseller partners in the brand's approved network.
Partners will be able to work closely with each of the resellers to develop integrated campaigns for different HP products. Channels include email marketing, digital advertising, direct marketing, telemarketing and events.
HP-approved reseller partners will have access to a dedicated marketing budget allocated on a quarterly basis. It will let the resellers request marketing services from Proximity London, which will then develop the campaigns.
Resellers can also consult an online guide to find out more about how they can use the different channels in their advertising.
Nicky Ayre, co-marketing manager at HP, said: "Over the past 18 months, HP has strived to provide clear and consistent communications to its partners. The appointment of Proximity London will allow our partners to have the same level of consistency in their customer communication."
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