Webroot has served notice on ISDG after completing a UK distribution review.
The cyber-security vendor is moving forward with three UK distributors - Distology, Northamber and Prianto - after concluding its distribution skyline had become too busy.
Theale-based ISDG's contract will cease on 17 June.
ISDG director Donald Wasterfall told CRN he "fully respected" Webroot's decision.
"Webroot made a decision to rationalise distribution as they felt the market was a bit crowded for the number of distributors they were working with," he said.
"Webroot has clearly communicated [the transition strategy] with all its resellers and MSPs and we will continue to offer 100-per-cent support until our notice period ends."
ISDG's partnership dates back to 2013, before it broke off from CDG.
Wasterfall said the duo's parting was a function of where ISDG plays in the vendor lifecycle. Its four remaining vendors are database security vendor GreenSQL, email security virtual appliance outfit Libra ESVA, hybrid cloud back-up vendor IASO and enterprise file sharing specialist Nomadesk.
"Webroot are a first-class, next-generation cyber-security product, which is why we started with them," he said. "This fully complies with our normal strategy of helping innovative new solutions into the market. Once that base is established, they expand and move on."
Webroot claimed in February that it now has 87,000 business customers after its Q2 sales grew "double digits".
Rick Yates, director of sales and marketing at Distology - which was the last of the three remaining distributors to come on board, last September - was unsurprised by the news.
"Webroot is rightfully seeing a lot of success in growing its UK business and all of the Webroot distributors had expected some consolidation in 2016," he said. "We will of course continue to help drive the Webroot business forward, with particular focus on the MSP space, which has seen a massive amount of growth for Webroot in the last 12 months."
Yuri Pasea, CEO of Prianto UK, said his firm has been focusing on getting Webroot into mid-sized resellers with £5m to £100m turnover.
"Once you get in there, and resellers see how different Webroot is, it has had a dramatic effect on our pipeline," Pasea said. "Four distributors to three is always a benefit, as it will be one fewer to compete against."
In a statement, a Webroot representative said: "At the moment we can only confirm that with effect from 17 June, 2016 ('effective date'), ISDG Limited is no longer a distributor for Webroot. Therefore, ISDG Limited and its employees/agents/representatives are not authorised to distribute any Webroot products starting from the effective date."
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