Mitvol said the deputy natural resources minister, Semyon Levi, signed an order Tuesday on his dismissal due to personnel cuts, but failed to hand him the document.
Mitvol said the document was signed in the absence of Minister Yury Trutnev, who on Tuesday told journalists in Irkutsk that he hoped the conflict between Mitvol and his boss, Vladimir Kirillov, would be resolved.
"I would very much like to speak to Trutnev," he said.
But a source in Rosprirodnadzor told RIA Novosti that Mitvol had been informed of his dismissal, which Mitvol denied.
In June, Kirillov dramatically reduced the role of his outspoken deputy, who had become known for high-profile campaigns against oil companies.
Mitvol was appointed deputy environment chief in April 2004 but came to international attention in late 2006 when he led a campaign against oil major Shell that resulted in a lucrative project being sold to Gazprom.
He brought the government's attention to damaging development work being carried out through the Sakhalin II oil and gas project in Russia's Far East, then led by Shell.
He has also spearheaded campaigns over ExxonMobil-led Sakhalin-I, the Kovykta gas field developed by TNK-BP, and a pulp mill next to Siberia's Lake Baikal.
Although in most cases the environmental damage Mitvol has highlighted, including deforestation, toxic waste dumping and soil erosion, has been well documented by environmental groups, the campaigns have often been portrayed in the Western media as part of the Kremlin's drive to bring key oil and gas assets back under its control.
Back in 2005, Mitvol gained publicity within Russia with widely reported cases involving pop diva Alla Pugachyova and electricity monopoly chief Anatoly Chubais over their country estates, which Mitvol said should be demolished because they had been built without planning permission in water-protection zones.
Reports that Kirillov planned to sack Mitvol emerged soon after the new chief's appointment in January this year.
One of Krillov's predecessors, Sergei Sai, tried to dismiss Mitvol in late 2006, but was overruled by the natural resources minister.