The news will come as a tremendous body blow to Zenith - the agency had handled the business for a decade and its loss comes just a week after the AA also stripped its £33 million media business out of the agency.
It also marks the first time that Starcom Motive and MediaVest, two Publicis-owned agencies, have won a collaborated pitch. The pair teamed up to defeat the incumbent and MediaCom in a three-way battle.
Initiative Media, Walker Media and Carat were all knocked out in the first round of the review, which was handled by the AAR.
Given the success of the pitch, it is expected that it could mark the start of further joint pitches and greater co-operation between Starcom Motive and MediaVest.
BT's head of media, Grant Millar, oversaw the pitch.
He said: "Starcom Media-Vest Group impressed with their progressive approach and were very inspirational in how to take things forward and extend what we are doing. Zenith has had the account for ten years and has delivered a high quality of service. We were surprised an agency was able to surpass its offer."
However, the loss of BT on top of the AA means that Zenith has lost one tenth of its annual billings and two of its most prestigious accounts in the space of seven days. Zenith does, however, retain the BT radio implementational planning and buying brief.
BT's strategic planning account held by PHD and the television buying brief held by The Allmond Partnership are not affected.