Viktor Chernomyrdin, who served under Boris Yeltsin as Russian prime minister during the 1990s, has died aged 72.
He came to political prominence as a Soviet minister for the gas industry, and then as head of Russia's new gas monopoly, Gazprom.
In 1992 he became prime minister, overseeing economic reform as well as talks to end the first Chechen war.
He took control of the Budyonnovsk hostage crisis in 1995, when rebels seized 1,500 people at a hospital.
During the five-day siege in southern Russia, Mr Chernomyrdin led negotiations with rebel leader Shamil Basayev while President Yeltsin was in the United States.
The crisis eventually came to an end when the prime minister promised a ceasefire in Chechnya.
Close to Yeltsin
But it is for his role in developing Russia into a market economy that Mr Chernomyrdin is most likely to be remembered.
Another former prime minister, Sergei Stepashin, said that in the mid-1990s, while Russia's economy was "languishing", Mr Chernomyrdin "pulled it out of the abyss".
He was Russia's longest-serving post-Soviet prime minister, in the post from 1992-98.
Viktor Chernomyrdin was very close to Boris Yeltsin and at one point was seen as a likely successor as president.
In November 1996, he became acting president for 23 hours while Mr Yeltsin was undergoing heart surgery.
When Vladimir Putin was elected president, Viktor Chernomyrdin left politics but served as ambassador to Ukraine from 2001-2009.
He was known to have been ill for some time but the Kremlin did not state the cause of his death.
The Itar-Tass news agency reported that Mr Chernomyrdin had been abroad for treatment.
Mr Putin, Russia's current prime minister, described him as a major political figure.
"He did a lot for the formation of our country, for the development of its economy, its social sphere, for strengthening its foreign policy."
President Dmitry Medvedev has told his chief of staff to organise the funeral, which is due to be broadcast live on Russian television on Friday.