Mediaforce beat off competition from Trinity Mirror's regional division Amra, and Archant representative Clacksons, to be awarded the £8m brief which also includes Sunday Life and the weekly community Telegraph titles.
The Belfast Telegraph's London sales are currently handled by an in-house team that has operated from Canary Wharf for the past nine years.
Moving its sales operation out of house is part of a larger restructuring plan by the publisher and Mediaforce's managing director, Lawrie Procter, said that he was hoping to take on the existing staff when the account moved over on 12 June.
Winning the sales contract for The Belfast Telegraph puts Mediaforce in poll position in the Northern Ireland market, where it already represents the Belfast Newsletter, following Johnston Press's aquisiton of the Local Press Group last September.
But some industry insiders are concerned that the addition of The Belfast Telegraph to the Mediaforce portfolio would lead to less competitive rates and compulsory purchasing of space in both titles.
Procter dismissed these claims and said that while there would be certain incidents when packaging would make sense it would be on a case-by-case basis.