The society's family law committee, which represents thousands of divorce lawyers, said the Bill's tortuous Commons stages had rendered it "a mess, creating more problems than it solves."
The move will make it easier for Labour, whose legal affairs spokesman Paul Boateng has already threatened to withdraw his party's backing, to instruct its MPs to vote against or abstain on the controversial measure when it reaches its Commons Third Reading next month.
Hilary Siddle, chairman of the society's family law committee said: "The Law Society supported the Bill's original principles, but changes to it have destroyed any consistent policy objectives."
Jonathan Evans, the Lord Chancellor's Department minister in charge of the Bill, last night wrote to MPs in all parties urging them to back the Bill, saying: "The current system of divorce lacks support on all sides of the House. It permits 'quickie' divorces in as little as three months on the basis of spurious allegations, without the couple having to give thought to the consequences of their actions. The Family Law Bill heralds the end of this damaging and nonsensical system."
Changes during the Committee Stage had strengthened the Bill's proposals on supporting marriage and protecting those who were most vulnerable, Mr Evans insisted.
Ministers are equally determined to maximise potential embarrassment for Labour if it directs its MPs to withhold support. Some of the changes, such as a 3-month quarantine period to facilitate reconcilation, which legal groups now object to, were made with active Labour support, while ditching the Bill would mean ditching pension splitting, enhanced protections against domestic violence while keeping the "quickie" divorce system.
Mr Evans said: "If the Labour party were to decide to oppose this Bill it would be nothing to do with the Bill and everything to do with party politics."
Labour would almost certainly face a backlash from some of its MPs as well, while the Conservative Family Campaign pointedly reaffirmed its support for the Bill last night.
Julian Brazier, Conservative MP for Canterbury, the campaign's chairman, attacked the Law Society's decision and said: "I am now firmly behind the Bill It represents an enormous improvement."
Thelma Fisher, director of National Family Mediation, said her organisation also remained committed to the Bill. "NFM considers that there are many aspects of the Bill that will better serve the needs of divorcing parents and, particularly, their children, by removing much of the acrimoney from the divorce process. The Law Society's withdrawal of support for the Bill is regretted."
But Mr Boateng called the society's decision "a body blow to a battered and beleaguered Bill whose days must now be numbered.''Reuse content