Weetos, Crunchy Bran, Alpen, Ready Brek and Seriously Oaty are all in the portfolio scooped by Nexus, along with the core Weetabix brand.
Nexus is planning a consumer PR push to promote the nutritional aspects of Weetabix products, which range from cereals to snack-bars.
PR work will support a major advertising campaign by DDB London, set to break next month.
Weetabix's chief executive is Ken Wood, who joined last year from dairy products giant Muller. Nexus used to handle consumer PR for Muller, work it lost to Chime-backed agency Resonate last month (PRWeek, 22 April).
Wood said: 'We have a big job to do in a short space of time and it is important to me personally to work with people who know how I operate.
It therefore made sense to transfer to Nexus.'
Nexus was hired to the Weetabix account without a competitive pitch.
Fees for the work are thought to stretch comfortably into the six-figure bracket.
The brief includes trade and corporate PR for the company, which was bought by Hicks, Muse, Tate & Furst - the venture capitalist now known as Lion Capital - in 2003.
The company entered the cereal-bar market in 2002 with the launch of Alpen bars. Products such as its Weetos Cereal Bar and Weetabix Chunkyfruit Bars have followed.
Weetabix was cited as Britain's top cereal brand 'by value' in a table published by The Grocer in January.
The loss of the Weetabix business is a blow to Biss Lancaster, which represented the brand since the late 1990s. Prior to Biss Lancaster, the account was held by Leedex.