Collective Legal Solutions Group is one of the largest businesses of its kind in the UK, dealing with inheritance-related legal matters.
Rod Bulmer, chief executive of consumer services at the Co-op, said: "This acquisition is a perfect fit for the Co-op, supporting our purpose of championing a better way of doing business.
"Our consumer services businesses - general insurance, funeral care and legal services - already serve millions of our members in communities across the UK and Collective Legal Solutions will further support that work."
Formed in 2007, Collective Legal Solutions provides wills and trusts, probate and lasting power of attorney.
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