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):

July 13, 2011

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 the discussion of the order issued by the Missouri Public Service Commission (“MoPSC”) on July 13, 2011, under Item 8.01 of this Current Report on Form 8-K. In the order, the MoPSC disallowed the recovery of all costs of enhancements related to the rebuilding of the Taum Sauk plant in excess of amounts recovered from insurance. As a result of this order, Ameren Corporation and its registrant subsidiary Union Electric Company, doing business as Ameren Missouri (“Ameren Missouri”), each expect to record a charge of $90 million, before taxes, in the third quarter of 2011 for the costs associated with these enhancements.

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 Form 10-Q for the quarterly period ended March 31, 2011, 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, 2010 (“Form 10-K”), of registrants Ameren Corporation and Amren Missouri for a discussion of Ameren Missouri’s requests to the MoPSC to increase its annual revenues for electric service.

As previously reported, Ameren Missouri filed a request in September 2010, with the MoPSC to increase its annual revenues for electric service. The request, as amended in April 2011, sought to increase annual revenues from electric service by approximately $210 million in the aggregate and was based on a 10.7% return on equity, a capital structure composed of 52.2% common equity, a rate base of $6.7 billion, and a test year ended March 31, 2010, with certain pro-forma adjustments through the true-up date of February 28, 2011.

On July 13, 2011, the MoPSC issued an order approving an increase for Ameren Missouri in annual revenues for electric service of $172 million, including $52 million related to the anticipated increase in normalized net fuel costs above the net fuel costs included in base rates previously authorized by the MoPSC in its May 2010 electric rate order. The revenue increase was based on a 10.2% return on equity, a capital structure composed of 52.2% common equity and a rate base of approximately $6.6 billion. The MoPSC also approved Ameren Missouri’s continued use of its existing 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. The MoPSC order granted Ameren Missouri’s request to continue to use its existing vegetation management and infrastructure cost tracker and the regulatory tracking mechanism for pension and postretirement benefit costs authorized by the MoPSC in earlier electric rate orders.

The order also allowed for the full recovery of investments for the Sioux plant scrubbers and related property taxes for the Sioux and Taum Sauk plants. However, the MoPSC disallowed the recovery of all costs of enhancements related to the rebuilding of the Taum Sauk plant in excess of amounts recovered from insurance.

Rate changes consistent with the MoPSC order are expected to become effective on or before July 31, 2011, with Ameren Missouri’s filing and MoPSC’s acceptance of conforming tariffs. Ameren Missouri is evaluating the MoPSC’s order and has not decided whether to seek rehearing of any aspect of the MoPSC’s order or subsequently appeal any aspect of the order. Moreover, the Missouri Office of Public Counsel and the intervenor parties to this rate 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 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 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:

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.

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: July 15, 2011