St Ivel has moved its entire pounds 9 million yellow fats business into Abbott Mead Vickers BBDO.
The move means the incumbent agency, D'Arcy, loses the St Ivel Gold, Utterly Butterly, Golden Churn and Vitalite accounts.
The company handed the business to AMV following the agency's successful launch of St Ivel's Carapelli brand last month.
The majority of St Ivel's advertising spend is on the Gold and Utterly Butterly brands. St Ivel asked both AMV and D'Arcy to generate ideas for the brands before moving the accounts.
Media planning and buying, which are through BMP Solutions in Media, are not affected by the change.
Cilla Snowball, AMV's managing director, said: 'These are great brands with huge potential. We pitched hard and we look forward to developing great work.'