The French government and the steelmaker had been waging high-stakes brinkmanship for weeks over the fate of two blast furnaces at a plant in Florange in northeastern France that ArcelorMittal said were no longer viable.
Last Friday Prime Minister Jean-Marc Ayrault reached an eleventh-hour deal in which ArcelorMittal would keep the 600 jobs and invest at least 180 million euros ($234 million) over five years at the plant. The fate of the furnaces was tied to a decision on an EU carbon capture project.
But Ayrault has struggled since to convince that the deal will prove effective and that Mittal would keep to his word.
On Thursday the European Commission disclosed that ArcelorMittal had withdrawn a request for funding under the EU's carbon capture site, but the company said this was due technical problems and was committed to the programme.
In the runup to the deal, the French government had threatened to nationalise the Florange site, sparking indignation in business circles.
In the letter, Mittal denounced the "anti-business language" used by the French government in the dispute and said he had been "confident" that authorities would not resort to a forced nationalisation.
He offered few new details about the deal, saying only: "We agreed with the government for the provisional stoppage and non-dismantling of the blast furnaces for six years."
He added that there would be no forced job cuts.