The news is a blow for the WPP-owned Portland because Kellogg is internationally aligned to WPP agencies. In the UK, J. Walter Thompson is responsible for part of its creative account while MindShare handles its media planning and buying.
Portland has also been rocked recently by the loss of clients such as COI Communications, UCI, Estee Lauder and The Telegraph Group.
In an attempt to shore up the company, WPP announced earlier this year that it was acquiring the outdoor independent Poster Publicity and would merge it with Portland.
A Kellogg spokeswoman said its contracts with agencies were confidential and would not comment.
Traditionally, Kellogg is not a big user of outdoor and spends less than 10 per cent of its total advertising budget on the medium. However, sources said it was looking to increase its outdoor spend.