The firm, whose brands include McVitie's and KP Foods, appointed H&K to the newly created account following a pitch in which eight agencies were invited to tender.
Corporate affairs was previously handled in-house but was disbanded by UB following its takeover by the Final Realm consortium last year.
This is not understood to have resulted in any redundancies, with all in-house corporate PR staff being redeployed in other departments.
Public affairs was previously handled by sole operator Michels Warren, who declined to repitch.
As part of H&K's brief, it will be UB's lead PR agency and co-ordinate the company's other agency relationships across Europe and the UK.
UK agencies include CirKle Communications, which handles trade PR, Revolver handling KP Snacks and Leedex Euro RSCG for KP Nuts.
H&K corporate and business communications MD Felicity Howard Allen said: 'It has not yet been decided whether we will have one strategy across Europe with some minor local tweaking or to look at it country by country.'
The H&K team reports to UB chairman and chief executive Malcolm Ritchie.
UB is still reviewing its consumer brief for McVities, which was left vacant following the resignation of Lexis PR from the account last month.
Lexis had represented the brand for four years.
The agency's CEO last month insisted the parting was amicable.
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