Italian Mario Scaramella, 36, has been a significant figure in an Italian espionage probe.
Mr Scaramella has said there are Soviet missiles in the Bay of Naples
He has advised the Italian Senate's Guzzanti committee, examining claims by Soviet defector Alexander Mitrokhin about KGB operations in the country.
As consultant to the committee the former environmentalist came into contact with Mr Litvinenko.
The pair would exchange information about Soviet-era activities in Italy.
Mr Litvinenko would also pass on information about what he thought his former spying colleagues were up to nowadays in Western Europe.
Mr Scaramella headed a body called the Environmental Crime Prevention Programme (ECPP), an organisation which tracked dumped nuclear waste, including Soviet nuclear missiles left over from the Cold War.
Between 2000 and 2002, Mr Scaramella was secretary general of that body.
In 2003 he made the unusual move from environmentalist to KGB expert for the Guzzanti committee's probe into post-World War II Soviet spying in Italy.
Many of his claims have been headline grabbing - including one in March 2005 when he said 20 Soviet nuclear warheads had been dumped by a USSR submarine in the Bay of Naples during the Cold War.
And last year he tipped off Italian police after saying he had been offered a suitcase of nuclear material for sale by a Ukrainian national.
At the time Mr Scaramella said he was investigating the activities of former KGB officers in the tiny Adriatic coast republic of San Marino.
He said he had been passed a document saying there were former KGB men in San Marino looking at selling nuclear military material. A garage in Rimini was raided and four men arrested.
He also said a tip-off from Mr Litvinenko helped prevent a potential attempt on the life of Paolo Guzzanti, a member of Silvio Berlusconi's Forza Italia party who is heading the probe into the Mitrokhin material.
The BBC's security expert Frank Gardner said: "The Scaramella file resembles a story from a spy novel.
"He is a 'self-styled' security expert - a change from being an environmentalist. He has been specialising in investigating the Russian intelligence service in Italy."
The Italian government has officially denied that Mr Scaramella works for Italy's secret service SISMI.