UPON A REHEARING EN BANC
Opinion
BAKER, J.In this rehearing en banc appeal from a majority opinion by a panel of this Court released on December 17, 1991,1 we reverse that portion of the judgment contained in a decree entered by the Circuit Court of the City of Roanoke (trial court) on February 19, 1991, that declared the child support reimbursement provisions of a Property Settlement Agreement (PSA) executed by David Allen Kelley (husband) and Marilyn Gibson Kelley (wife), which had been ratified, affirmed and incorporated by reference in a decree of divorce entered by the trial court on September 23, 1985, to be “null and void ... against public policy,” and unenforceable. For the reasons that follow, we hold that after the expiration of twenty-one days immediately following the entry of the 1985 divorce decree, except to modify the amount of child support, the trial court lacked jurisdiction to alter the terms of the PSA or the decree.
The marriage of husband and wife was dissolved by decree of the trial court entered on September 23, 1985. Two children were born of the marriage. On April 29, 1985, the parties executed a PSA that was ratified, affirmed and incorporated by reference into the divorce decree. Relevant to this appeal, the PSA contained the following provisions:
*95The parties hereto agree, in consideration of Husband relinquishing all of his equity in the jointly-owned marital home,2 that Husband shall never be responsible for payment of child support. The party of the second part covenants and agrees never to file a petition in any Court requesting that David Allen Kelley be placed under a child support Order because Marilyn Gibson Kelley has accepted all of David Allen Kelley’s equity in lieu of requesting child support.
In the event Marilyn Gibson Kelley should ever petition any Court of competent jurisdiction for support and maintenance of Joshua Caleb Kelley and/or Joanna Caren Kelley, and should a Court grant any such child support award, the said Marilyn Gibson Kelley hereby covenants and agrees to pay directly to David Allen Kelley, any amount of support that he is directed to pay to any party. In other words, Marilyn Gibson Kelley is agreeing to hold harmless David Allen Kelley from the payment of any amount of child support, regardless of the circumstances under which he is paying the same.
On October 31, 1990, husband filed a motion requesting that the trial court order definite and specific visitation rights, and enjoin wife from “harassing and/or communicating with him at any place.” Wife countered by asking the trial court to order husband to pay support for their children. Pursuant to the PSA that had been incorporated into the divorce decree, husband moved the trial court to order wife to reimburse husband for any child support monies he may be ordered to pay.
On February 19, 1991, the trial court ordered husband to pay wife $604 monthly as support for the two children, and denied husband’s motion for reimbursement after declaring the reimbursement provision to be null and void as contrary to public policy. Husband concedes that, notwithstanding the passage of more than twenty-one days immediately following the entry of the divorce decree, the trial court has the limited power to modify its prior decrees relating to the amount of child support husband may be required to pay. Rook v. Rook, 233 Va. 92, 95, 353 S.E.2d 756, 758 (1987). In fact, the PSA obviously anticipated that wife might subsequently petition the court for child support. However, husband contends that because the terms *96of the PSA constituted a valid, binding contract that was incorporated into the divorce decree, the trial court did not have authority to modify the terms of the PSA or the decree ordering compliance with its terms.
In Rook, while still living together, a married couple executed a PSA in contemplation of a separation. Id. at 93, 353 S.E.2d at 757. Generally, such agreements are void as against public policy because they tend to encourage divorce. See Chattin v. Chattin, 245 Va. 302, 310, 427 S.E.2d 347, 352 (1993). The trial court, by a vinculo decree, dissolved the Rook marriage and incorporated the PSA into the decree. In a subsequent contempt proceeding for failure to comply with the terms of the decree, the husband contended that the PSA was void ab initio and unenforceable as against public policy because it facilitated separation and divorce. The circuit court agreed. Rook, 233 Va. at 93, 353 S.E.2d at 757. On appeal, Rook’s former wife, relying upon Rule 1:1, asserted that the trial court did not have jurisdiction after twenty-one days to modify the divorce decree. The Supreme Court agreed with the wife’s argument.
Where the trial court affirms, ratifies and incorporates such an agreement into its decree by reference, “ ‘it shall be deemed for all purposes to be a term of the decree, and enforceable in the same manner as any provision of such decree.’ ” Id. at 94, 353 S.E.2d at 757 (quoting Code § 20-109.1). Upon the expiration of twenty-one days, the judgment of the trial court may not be modified unless the judgment is void as having been obtained by extrinsic or collateral fraud, or entered by a court that did not have jurisdiction over the subject matter or the parties. Id. at 95, 353 S.E.2d at 758; see also Jones v. Willard, 224 Va. 602, 607, 299 S.E.2d 504, 508 (1983); Cofer v. Cofer, 205 Va. 834, 836-37, 140 S.E.2d 663, 665-66 (1965).
Notwithstanding Rule 1:1, a void judgment may be attacked in any court at any time, directly or collaterally. Rook, 233 Va. at 95, 353 S.E.2d at 758. However, a void judgment is one that has been procured by extrinsic or collateral fraud, or entered by a court that did not have jurisdiction over the subject matter or the parties. See id.; see also Owusu v. Commonwealth, 11 Va. App. 671, 672, 401 S.E.2d 431, 431 (1991).
In the case before us, there is no claim that, when the PSA was executed and the divorce decree entered, the parties were not sui juris, or that the PSA was obtained by extrinsic or collateral fraud. Moreover, *97there is no claim that the circuit court lacked jurisdiction of the subject matter and the parties. See Owusu, 11 Va. App. at 672-73, 401 S.E.2d at 431. The divorce decree that incorporated the PSA orders that “the parties are directed to strictly abide by the terms of said agreement.” The present challenge to the validity of the agreement and divorce decree “could and should have been raised before the divorce decree was entered or within 21 days thereafter.” Rook, 233 Va. at 95, 353 S.E.2d at 758; Rule 1:1. After the expiration of twenty-one days from the date the divorce decree was entered, the trial court lost jurisdiction of the case, except for the limited purposes of revising child custody and support. Rook, 233 Va. at 95, 353 S.E.2d at 758; Code § 20-108.
We hold that on February 19, 1991, the trial court was without jurisdiction to enter a decree that effectively declared the reimbursement portion of the divorce decree to be a void judgment. Accordingly, we hold that the trial court erred when it declared that portion of the PSA to be invalid as against public policy.
Wife argues that Rule 5A:18 precludes this Court from applying Rule 1:1 because husband failed to raise the issue before the trial court. However, husband premised his entire argument before the trial court upon the enforcement of the PSA. Because the PSA was incorporated into the divorce decree and was a term of that decree, our consideration of the void judgment issue is appropriate.
When the circuit court entered the divorce decree by affirmation, it approved the validity of the PSA. When it ratified and incorporated its terms into the divorce decree and directed the parties to abide by its provisions, the parties were obligated to observe its terms. See generally Owen v. Owen, 14 Va. App. 623, 419 S.E.2d 267 (1992). Upon review of this contract, entered into by competent adults for a valuable consideration, which clearly expresses the intent of the parties, and approved by the divorce chancellor, we cannot say that it is null and void on its face as held by the trial court. “Valid marital agreements are favored and will be enforced by divorce courts when incorporated in a decree which orders the parties to comply with its terms.” Doherty v. Doherty, 9 Va. App. 97, 99, 383 S.E.2d 759, 760 (1989).
For the reasons stated, we reverse and remand this case to the trial court for further consideration consistent with this opinion.
Reversed and remanded.
*98Moon, C.J., Barrow, I, Duff, J.,* Willis, X, and Bray, X, concurred.
Kelley v. Kelley, 13 Va. App. 424, 412 S.E.2d 465 (1991).
When the transfer of husband’s interest was conveyed to wife, pursuant to the PSA, the equity in the marital home was $40,500.
Judge Duff participated in the hearing and decision of this case prior to the effective date of his retirement on September 1, 1992, and thereafter by designation pursuant to Code § 17-116.01.