Lidington joined Research International in 2004 after working as a partner with management consultancy Quadrangle.
He has 28 years' exprerience in market research, starting client side at Manpower Services Commission and then Fiat before moving to BJM and PAS Research.
In addition, he co-founded The Business Research Unit and was elected to the MRS Council prior to joining Research International.
Research International would not confirm whether Lidington is leaving to take up another position elsewhere.
Roger Banks, Research International group chief executive, is taking on the role of interim chief executive for the UK business, in addition to his existing responsibilities, until a successor is found.
Banks said: "Simon made a valuable contribution to our thinking, especially in relation to our commitment to being a client-centric organisation, which delivers strategic insight that enables clients around the world to achieve their goals.
"These principles will continue to be at the heart of our approach going forward."
Research International is a custom market research network, part of WPP's information and consultancy division The Kantar Group.
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