Mr Litvinenko is renowned for his critique of Russia's security services
News - Profile: Alexander Litvinenko
Alexander Litvinenko, the former Russian security agent fighting for his life in a UK hospital after allegedly being poisoned, has been a fierce critic of Vladimir Putin since before he became president in 2000.
Mr Litvinenko is thought to have been close to journalist Anna Politkovskaya, another opponent of the Kremlin who was shot dead last month, and said recently he was investigating her murder.
It was after being handed documents apparently relating to the case that he was taken ill more than two weeks ago.
But he is perhaps best known for a book in which he alleges that agents co-ordinated the 1999 apartment block bombings in Russia that killed more than 300 people.
He now appears to have fallen victim to the kind of plots which he wrote about.
Mr Litvinenko, 43, first became a security agent under the Soviet-era KGB, rising to the rank of lieutenant-colonel in its later incarnations.
He is reported to have fallen out with Vladimir Putin, then head of the security service, in the late 1990s, after failing in attempts to crack down on corruption within the organisation.
In 1998, he first came to prominence by exposing an alleged plot to assassinate the then powerful tycoon Boris Berezovsky, who himself now lives in self-imposed exile in the UK.
He was subsequently arrested on charges of abusing his office and spent nine months in a remand centre before being acquitted.
In 1999 he wrote Blowing up Russia: Terror from Within, in which he accused the current Russian security service, the FSB, of carrying out several apartment house bombings in 1999 that killed more than 300 people.
The attacks, which Moscow blamed on Chechen rebels, helped swing public opinion behind Russia's second war in the breakaway republic.
Complaining of persecution, in 2000 Mr Litvinenko fled to the UK where he sought, and was granted, asylum.
But after settling in an unnamed London suburb, the former spy continued to behave as if on the run, constantly changing his contact details.
The Times newspaper reported that over the summer someone tried to push a pram loaded with petrol bombs at his front door.
Appearing alongside high-profile opponents of President Putin, he has continued to make allegations about his former bosses.
Perhaps most notably, he alleged that al-Qaeda number two Ayman al-Zawahiri was trained by the FSB in Dagestan in the years before 9/11.