BT Wholesale Markets provides its telecoms and broadband infrastructure to other telecoms suppliers as well as direct to businesses.
H&K previously handled the B2B and broadband part of the account while Pirate held the corporate media relations brief.
It is understood that fees on the combined account have been reduced from £750,000 to around £250,000 a year.
A BT Wholesale Markets spokeswoman said the company felt it would be more effective to have a single agency handling its entire communications.
