The head of Russia's GRU military intelligence agency, General Igor Korobov, has died aged 62, Russia's defence ministry says.
Gen 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 Russian ex-spy Sergei Skripal and his daughter.
Gen 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 4 March 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 denies the allegations. The UK and its Western allies expelled more than 150 Russian diplomats in retaliation for the attack.
Vice-Admiral Igor Kostyukov, who has commanded Russian forces in Syria, will serve as interim GRU chief. He had already taken over while Gen Korobov was ill, military sources told Russian media.
The appointment of a permanent chief is a matter for President Vladimir Putin.
The GRU, or Main Intelligence Directorate, is the intelligence arm of the Russian military, tasked with carrying out undercover international operations.
Gen Korobov had received the Hero of Russia medal - the state's highest award.
In December 2016 the US added Gen Korobov to its list of senior Russian officials subject to sanctions, accusing him of involvement in computer hacking.
Other Western sanctions target Russians accused of helping the separatist insurgents in Ukraine.
Russia has two other main spy organisations: the Federal Security Service (FSB), mainly involved in internal security, and the Foreign Intelligence Service (SVR), whose role is similar to that of Britain's MI6.
GRU's year of notoriety
The BBC's Steve Rosenberg in Moscow writes:
Igor Korobov's career in the Soviet, then Russian military, spanned more than 40 years.
He joined military intelligence in the mid-1980s and rose through the GRU ranks to a key position, overseeing strategic intelligence-gathering.
In 2016 he was given the top job: agency chief.
Under Korobov, and his predecessor Igor Sergun, the GRU became the Kremlin's spy agency of choice for a series of high-profile and highly controversial operations. The GRU has been linked to Russia's military operation in eastern Ukraine, to Moscow's meddling in the US election and to a failed coup in Montenegro.
The defence ministry described Korobov as a "wonderful person". But 2018 has been less than wonderful for the agency he headed. A series of botched operations - most notably the Salisbury poisoning - has thrust the GRU into the limelight and raised questions about its methods and activities.
Korobov reportedly died "after a long, extended illness". Did he fall ill, as rumours suggest, after a dressing-down from the president? I can't confirm that.
But what's clear is that if you take on the role of GRU chief, don't expect a carriage clock and a happy retirement at the end of it. Korobov's predecessor, Igor Sergun, also died while in office.
Gen Korobov 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 is highly secretive - its total staff is not known, nor is its organisational structure known in detail.
The GRU was involved in undercover operations in Ukraine - including Russia's annexation of Crimea in 2014 - and allegedly in the computer hacking during the 2016 US presidential election.
The GRU includes Spetsnaz special forces - crack military units - who have fought in the Syrian war, helping President Bashar al-Assad's troops, and against separatist rebels in Chechnya. In the Soviet period they fought against Western-backed guerrillas in Afghanistan.
In 2015 President Putin admitted that GRU units had been deployed in Crimea shortly before the peninsula was annexed.
Among the GRU's tasks are: agent-running, sabotage, hi-tech eavesdropping and reconnaissance, the government daily Rossiyskaya Gazeta reported.
The Spetsnaz logo is a black bat with wings spread out across the globe.
According to the Russian news website Meduza, GRU officers get three years of specialist training, which includes cybernetics, foreign languages, geopolitics, use of codes and other elements of espionage.