Leeds' members will vote on the tie-up at a meeting on 11 December. If two thirds vote in favour, a confirmatory meeting will be held on 8 January 2007, with the merger coming into effect later that month.
It will be the latest in a string of Co-op mergers. Earlier this month Ilkeston Co-operative merged with Midlands Co-operative, and last month members of Sheffield Co-operative Society voted to join up with United Co-operatives.
Last year Oxford, Swindon & Gloucester merged with West Midlands Co-op to create Midcounties Co-operative Society, while a merger between Ipswich & Norwich Society and Colchester & East Essex Society led to the East of England Co-operative Society being formed.
Co-op numbers have fallen from more than 1,000 at the end of the 19th century to less than 40 now.
The United-Leeds merger was necessitated by supermarket competition and rising costs, said Leeds Co-op president David Schofield. "Against that background, we believe the best way to build further on our achievements is through a merger."
United Co-op has annual sales of more than £2bn and 530 grocery stores. Leeds Co-op has a turnover of £76m, with 22 food outlets.