Tretyakov, who was deputy head of intelligence at the Russian mission at the United Nations for five years, has made a series of revelations since defecting to the USA in October 2000 after a period as a double-agent.
The former senior spy head who now lives incognito in the USA has claimed that a Swedish diplomat was among the Russian security services' sources recruited at the United Nations in New York.
"He was a brilliant guy. He was very valuable," Tretyakov told the Expressen daily.
The newspaper has gained access to classified Swedish Security Service documents which confirm that an investigation was opened against the Swedish diplomat in question but that the case was closed after three months due to a lack of evidence.
"For it to be classified as spying I have to prove that information was revealed that was detrimental to national security," said prosecutor Tomas Lindstrand to the newspaper.
The diplomat was summoned back to Sweden in 2002 when the suspicions over his actions came to light. He remains employed within the foreign ministry.
According to Tretyakov, the documents provided by the Swedish diplomat, who was given the codename Sylvester, include internal EU communications that diplomats were prohibited from passing on, "especially to the Russians."
"He gave us confidential EU documents... He was telling us what we were asking him," the former spy chief told Expressen.
According to Tomas Lindstrand, it is not clear that the Swedish diplomat had access to any documents that could have harmed Sweden's national security. He confirmed to the newspaper that handing over confidential documents is not in itself an offence according to current legislation.
But Tretyakov argued to the newspaper that the diplomat clearly knew that he had "crossed the line" of what is acceptable by handing over the documents and also, he alleges, for accepting gifts.
The Swedish former UN diplomat at the centre of the revelations confirmed in an email to Expressen that he had "some contact with Russian representatives" during his time in New York, but claimed that the former spy chief's information was misleading.
In January 2008 Sergei Tretyakov gave a series of interviews to publicise a book about his experiences. In the exposé he made several claims about links with US, Canadian and British politicians and argued that the Russian intelligence service were just as active now as in the Cold War era.
The SVR is the name of Russia's foreign intelligence agency and is the successor of the FCD arm of the KGB, which ceased operation in December 1991. The organization works with the Russian military intelligence organization GRU.