A consortium of five investors will launch a bid to demutualise life and general insurer NFU Mutual next week.
The unnamed consortium includes two bankers and the finance director of a large insurance company.
A spokesman for the consortium said: "The first approach will be to offer a friendly agreed deal. If this is not accepted, we will go to the members and we believe they are likely to accept."
Analysts estimate that the insurer is worth £3bn.
A former director of NFU Mutual, John Murray, is plotting a rival offer. It was Murray who initially approached the five City investors in the hope of launching a bid, but talks with the consortium fell through two months ago.
Murray said: "I know nothing about what the consortium will do. It is well known that the bid was my idea and I am still interested in pursuing it."
Murray is understood to be in talks with one "major financial institution" to provide financial backing for his bid, which will be tabled "within months".
Both deals would take the mutual into private corporate ownership and would, according to the bidders, generate a windfall of £20,000 to £25,000 for members. The company has 850,000 policyholders, but less than half are thought to be eligible for windfalls.
A conversion resolution would have to be voted on by at least half of the members and receive at least 75% support from those who vote.
An NFU Mutual spokeswoman said the insurer had not yet received any bid and would not welcome one. She added: "We are committed to mutuality."
NFU Mutual has been criticised by the consortium of five bidders for moving away from its specialist farming arm and selling general insurance products to non-farming customers through its subsidiary Avon Insurance.
The consortium's spokesman said: "NFU is selling to non-agricultural people at a cut-price premium.
"It is eroding money that could be available for distribution by selling outside its sector."
In 2000, NFU Mutual group generated total gross written premiums of £656m, £504m of which came from NFU Mutual and £152m from the non-farming arm, Avon Insurance.
