The decision on so-called "pension splitting" will take many Conservative peers by surprise. Labour, whose social security spokeswoman, Baroness Hollis, is promoting the amendments, had hoped for a free vote.
Lady Hollis has put down amendments for the change to be enshrined in the Family Law Bill that reaches the second day of the report stage today, leaving the details to a later date.
The proposal would enable couples to divide pensions - often one of the most substantial assets of a marriage - at the time of divorce. Currently, a divorced wife is at risk of receiving nothing in her old age.
Similar amendments drew all-party support during the Bill's committee stage, and the apparent sympathy of the Lord Chancellor, Lord Mackay. But the Treasury and the Department of Social Security intervened, arguing that the reform would be both expensive and difficult to implement.
While a Green Paper on the topic has been promised, Lady Hollis is determined to press on with securing legislation, which is backed by the pensions industry, the Law Society and even the Mothers' Union.
Lady Hollis said: "This issue isn't party political at all. Loyal wives and mothers should not be forced into penury by being traded in a for a newer model."
A further revolt is looming later today when peers vote on an amendment proposed by the former Conservative party vice-chairman, Lady Young, to reintroduce fault-based divorces into the Bill. Lord Mackay is expected to win but only with Opposition backing.
Meanwhile, seven members of the influential executive of the Tory 1922 committee have now expressed opposition to the Bill, which goes back to the Commons next month.Reuse content