The move will increase speculation that Flextech's owner, Telewest, is planning to sell the division ahead of a merger with NTL. A number of broadcasters including Hallmark, Viacom and RTL have emerged as potential bidders.
Howe has worked at the company for 13 years. In a statement, the company said that he was leaving "to pursue new challenges". James Wildman, executive sales director, will now report directly to Lisa Opie, the managing director of Flextech Television.
Howe said: "After nearly 13 years working in the company it's time to move on. I am proud of the strength of the Flextech ad sales business which culminated in the creation of IDS. I leave one of the strongest, most dynamic sales teams in the business in good hands."
Opie said: "Mark has made a huge contribution to the success of IDS, helping to make it one of the most successful advertising sales houses in the UK. I'd like to thank him for all his hard work."
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