UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (Date of earliest event reported): December 12, 2012

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Reference is made to Note 2 – Rate and Regulatory Matters to our financial statements under Part I, Item 1. Financial Statements and to Results of Operations and Outlook under Part I, Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations, each in the Quarterly Report on Form 10-Q for the quarterly period ended September 30, 2012 (“Third Quarter Form 10-Q”), and to Note 2 – Rate and Regulatory Matters to our financial statements under Part II, Item 8. Financial Statements and Supplementary Data and to Results of Operations and Outlook under Part II, Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations, each in the Annual Report on Form 10-K for the year ended December 31, 2011 (“Form 10-K”), of registrants Ameren Corporation and Union Electric Company, doing business as Ameren Missouri (“Ameren Missouri”), for a discussion of Ameren Missouri’s request to the Missouri Public Service Commission (“MoPSC”) to increase its annual revenues for electric service.

On December 12, 2012, the MoPSC issued an order (“Order”) approving an increase in Ameren Missouri’s annual revenues for electric service of approximately $260 million, including approximately $84 million related to the increase in net fuel costs above the net fuel costs included in base rates previously authorized by the MoPSC in its July 2011 electric rate order (“July 2011 Order”) and approximately $80 million related to the recovery of costs associated with energy efficiency programs under the Missouri Energy Efficiency Investment Act. The remaining increase of approximately $96 million approved by the Order was for other non-fuel costs, including $10 million for increased pension and other post-employment benefit costs and approximately $6 million for increased amortization of regulatory assets. The revenue increase was based on a 9.8% return on equity, a capital structure composed of 52.3% common equity and a rate base of approximately $6.8 billion.

The MoPSC also approved Ameren Missouri’s continued use of its fuel adjustment clause, or FAC. The FAC provides for the adjustment of electric rates three times per year for a pass through to customers of 95% of changes in fuel and purchased power costs net of off-system revenues, subject to MoPSC prudency review. Transmission charges that had previously been included in the FAC remain in the FAC. Transmission revenues previously included in base rates will be included in the FAC prospectively. This change resulted in the portion of the rate increase attributed to net fuel costs being reduced, and the portion attributed to other non-fuel costs being increased, by $33 million as compared to base rates authorized in the July 2011 Order. This change in regulatory treatment will have no immediate impact on earnings. Further, the Order clarified that changes in costs for activated carbon, limestone and urea are included in the FAC.

The Order also granted Ameren Missouri’s request to continue to use its existing vegetation management and infrastructure inspection cost tracker and the regulatory tracking mechanism for pension and other post-employment benefit costs authorized by the MoPSC in earlier electric orders. In addition, the Order established a storm restoration cost tracking mechanism to facilitate the recovery in future rate cases of storm costs that vary from those included in rates, and allowed retention of the refund received in June 2012 from Entergy Arkansas, Inc. related to a power purchase agreement that existed prior to the implementation of the FAC. However, the MoPSC did not approve Ameren Missouri’s request for plant-in-service accounting treatment for assets placed in service between rate cases or recovery of its 2011 severance costs.

Rate changes consistent with the Order are expected to become effective on or before January 2, 2013, with Ameren Missouri’s filing and MoPSC’s acceptance of conforming tariffs. Ameren Missouri is evaluating the Order and has not decided whether to seek rehearing of any aspect of the Order or subsequently appeal any aspect of the Order. Moreover, the Missouri Office of Public Counsel and the intervenor parties to this case may similarly seek rehearing or subsequently appeal any aspect of the Order. Ameren Missouri cannot predict whether any such application for rehearing or appeal will be filed, or the outcome if so filed.

Forward-Looking Statements

Statements in this report not based on historical facts are considered “forward-looking” and, accordingly, involve risks and uncertainties that could cause actual results to differ materially from those discussed. Although such forward-looking statements have been made in good faith and are based on reasonable assumptions, there is no assurance that the expected results will be achieved. These statements include (without limitation) statements as to

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future expectations, beliefs, plans, strategies, objectives, events, conditions, and financial performance. In connection with the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995, we are providing this cautionary statement to identify important factors that could cause actual results to differ materially from those anticipated. The following factors, in addition to those discussed under Risk Factors in the Form 10-K and the Third Quarter Form 10-Q and elsewhere in this report and in our other filings with the Securities and Exchange Commission, could cause actual results to differ materially from management expectations suggested in such forward-looking statements:

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Given these uncertainties, undue reliance should not be placed on these forward-looking statements. Except to the extent required by the federal securities laws, we undertake no obligation to update or revise publicly any forward-looking statements to reflect new information or future events.

This combined Form 8-K is being filed separately by Ameren Corporation and Union Electric Company (each a “registrant”). Information contained herein relating to any individual registrant has been filed by such registrant on its own behalf. No registrant makes any representation as to information relating to any other registrant.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, each registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized. The signature for each undersigned company shall be deemed to relate only to matters having reference to such company or its subsidiaries.

Date: December 17, 2012