Speed, which repitched for the business, had worked on Virgin’s business PR since its formation in 2009. Previously to that Virgin, then known as NTL:Telewest, worked with Rainier, which was later merged with other agencies to form Speed.
The brief, understood to be worth a significant six-figure sum, covers Virgin’s broadband and telecoms business services.
Bell Pottinger Business Brand MD Kevin Read told PRWeek the brand has "one of those great stories to tell. Virgin Media Business is a major challenger in the marketplace and is a massive growth area as businesses adapt to the digital world".
He added that the agency would bring "new creative thinking into things" to make Virgin a stronger challenger in the market.
The agency will lead integrated campaigns including media and analyst relations, thought leadership and social media.
The split follows the sale of Virgin Media to international media company Liberty Global earlier this year.
Former Vodafone enterprise director Peter Kelly joined Virgin Media Business as managing director in September.
Virgin previously announced it is set to cut 600 posts as part of a push to be more "agile and efficient" post-merger.
Tom Mockridge, the newly installed chief executive officer of Virgin Media, said: "Like organisations across the public and private sector, Virgin Media is making sure it has the structure it needs to meet the needs of its customers.
"These proposals are designed to take advantage of the opportunities that come with being part of the world's largest cable operator and create an organisation that's fit for growth."