Gill Bennett, the Foreign Office's chief historian, is to spend a week in Moscow next month delving into files which could shed significant light on the 74-year-old document, which linked the Soviet leadership with the Labour party.
The row which ensued when the letter became public was partly blamed for causing the landslide election defeat of Labour's first prime minister, Ramsay MacDonald, in 1924.
KGB documents declassified in Russia in January reportedly proved the letter was a forgery commissioned by British intelligence.
A month later, Robin Cook, the Foreign Secretary, authorised Ms Bennett to trawl through British secret service files to try to solve the mystery. But she realised that without access to the Russian files, the inquiry would be incomplete.
Yuri Fokine, the Russian ambassador in London and a former aide to the Soviet foreign minister Andrei Gromyko, has been instrumental in persuading Russian officials to grant access.
Ms Bennett said yesterday: "My researches are primarily based on the records of the intelligence agencies. But in order to get as full a picture as possible and produce as authoritative picture as possible, I'm going to see what the Russians have got in their archives."
Yuri Fokine had been "very helpful" in suggesting places which might provide useful leads, she said. "These days the Russians are much more open with their archives, so I am hoping to see a range of different ones including those of the former Russian intelligence services."
She will be assisted by Tony Bishop, the Foreign Office's principal Russian interpreter for 30 years until his retirement earlier this year.
The letter, purportedly by Grigori Zinoviev, a leading Bolshevik, was leaked to the Daily Mail in 1924 under the headline "Civil War Plot By Socialists' Masters. Moscow Orders To Our Reds."
It asked the British Communist Party to "mobilise" what it called "sympathetic" contacts inside the Labour party.
The newly-opened KGB files reportedly show that MI6 commissioned the letter from an agent in Riga, Latvia, to prevent MacDonald winning a second term in office.
Experts differ in whether they believe the letter was a forgery or not. Ms Bennett, who was made an OBE in the recent honours for her work as official historian, said she personally believed the truth may never be known for certain.
But she said: "Who wrote it is much less interesting than the point about what use was made of it when it got here.
"How was it released to the press here and why did it cause such an uproar?"
She hopes to deliver her final memorandum on the letter by the end of the summer.
"What I can say with absolute confidence is the memorandum will include a lot of new material never before seen."
The surviving British files on the matter had been closed to all but members of the intelligence services until Robin Cook ordered the new inquiry by Ms Bennett earlier this year.Reuse content