March 23 (Reuters) - Moody’s downgraded on Tuesday the debt ratings of Exxon Mobil Corp by a notch to Aa2 from Aa1, saying that the company’s policy to maintain its large dividend will slow down its debt-reduction process. The coronavirus pandemic hammered energy prices and reduced the value of the U.S. oil producer’s shale gas properties by more than $20 billion, with the company posting a historic annual loss for 2020. The ratings agency assigned a “stable” outlook for Exxon’s Aa2 rating in its second downgrade for the company in almost a year. “The company is prioritizing debt reduction through capital spending restraint and free cash flow generation going forward, but by maintaining its large dividend the progress will be slow and subject to the uncertainties regarding commodity prices, downstream earnings recovery and asset sales,” said Pete Speer, Moody’s senior vice-president. Moody’s forecasted Exxon’s debt levels to remain high through the end of 2022 and said that could also leave the company vulnerable to environmental and social risk, as countries look towards policy changes to limit carbon dioxide emissions. Last week, activist-investor Engine No. 1 launched a proxy fight seeking to unseat four Exxon directors and install its own candidates that it says would increase profit and accelerate a shift to cleaner fuels. Shareholders will have their say on May 26. (Reporting by Nandakumar D and Shubham Kalia in Bengaluru; Editing by Sherry Jacob-Phillips)