The request comes less than two months after Somerfield entered final negotiations with M&C Saatchi to move its £6m creative account from McCann-Erickson.
An email sent from its procurement department on Tuesday said: "We are currently doing a costing review exercise of our creative advertising agency, and would like to give you the opportunity of putting forward your company's cost if we were to use you as our vendor."
One agency source said: "It's ridiculous. We're having nothing to do with it."
The document outlined the brief to agencies. It included the "development and creation and national launch of a new advertising campaign with the continuing development of the campaign to drive brand awareness and sales".
A spreadsheet was provided to fill out hourly rates, work per month and the percentage of time that staff would spend on the Somerfield account.
"This is just a benchmarking exercise to review our costs and make sure the contracts are competitive. There is nothing suspicious about it," Susan Davison, a member of the STEPS Project, part of Somerfield's procurement division, said.
A Somerfield spokesman said the request would have no effect on M&C Saatchi's hold on the account.
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