The head of Russian military intelligence agency GRU, General Igor Korobov, has died at the age of 62, Russia's defence ministry says.
General Korobov, who took up the post in 2016, is said to have died after "a serious and long illness" on Wednesday.
The GRU was this year linked to a nerve agent attack in Britain on ex-Russian spy Sergei Skripal and his daughter, according to a BBC report Thursday.
General Korobov is understood to have faced criticism by Russian officials over the failure of the operation.
The attack on Mr Skripal, 66, and his 33-year-old daughter, Yulia, in Salisbury on March 4 led to them requiring weeks of hospital treatment.
The poisoning is alleged to have been carried out by suspects Alexander Mishkin and Anatoly Chepiga, both identified by British authorities as GRU operatives, and "almost certainly" approved by the Russian state.
Russia, however, denies the allegations.
General Korobov joined the Russian military in 1973 and worked with intelligence from 1985, receiving numerous awards and medals, according to Russia's state news agency Tass.
Tass tweeted the news of his death with an image of General Korobov in a grey military jacket.
He was described by Russia's defence ministry on Wednesday as "a wonderful person, a faithful son of Russia and a patriot of his homeland," Reuters news agency reported.
The ministry did not provide any further details about his death.
The GRU, or Main Intelligence Directorate, is the intelligence arm of the Russian military tasked with carrying out undercover international operations.
It has allegedly been involved in undercover operations in Ukraine - after Russia's annexation of Ukraine's southern Crimea peninsula in 2014 - and is accused of direct involvement in the hacking of the 2016 US presidential election.