Sotheby's estimates 320-page document detailing composer's second symphony will fetch up to £1.5m at auction in May

An autographed manuscript of Rachmaninov's greatest symphony that was assumed lost for nearly a century is to be sold at auction in London.

Sotheby's will announce that it will sell what is a truly remarkable musical rarity, the 320-page manuscript for the composer's second and most-loved symphony, full of his revisions, crossings out and annotations.

"It is a magnificent manuscript," said Stephen Roe, Sotheby's worldwide head of books and manuscripts. "Everything is here, the whole story, the whole great Russian novel that is the second symphony."

It will be the second time Sotheby's has attempted to sell the manuscript. In 2004, to some astonishment, it emerged that the manuscript, which had long been thought lost, had in fact been in the collection of an unnamed European collector.

Sotheby's announced it was to be auctioned with an estimate of between £300,000-£500,000 but the sale was abandoned after relatives of the composer, who control the Rachmaninov estate, launched a legal battle claiming ownership.

That case was then settled amicably according to Sotheby's, and the manuscript was sold privately to the Tabor Foundation, which then placed the manuscript on deposit at the British Library.

Now it is back, this time with an estimate of £1m-£1.5m.

Rachmaninov's second is his greatest work and was completed in Dresden by mid-January of 1908. Rehearsals were directed by his friend and cousin Alexander Siloti and Rachmaninov himself conducted the first performance in St Petersburg on 26 January 1908. It is this manuscript that experts believed he used.

Sotheby's concede it is not 100% certain and that he could have conducted from memory or used a copyist's score but, it adds: "What seems highly likely is that Rachmaninov revised the orchestration in the light of these performances, so that his original orchestration is preserved only in this score."

Roe said: "It shows the workings and how it reached its first publication, it reveals his original thoughts and revised thoughts. There is no substitute for looking at a composer's autograph – you can see various layers of working on it and various layers of realisation of his ideas."

There are changes and reworkings, heavy in places, made in ink, pencil and red and blue crayon – used the way we might use coloured marker pens today.

You get a sense of passion, said Roe, but "he has a very neat and clear hand and it is almost dispassionate in a way. It is a very measured and clearly laid-out score, very neat and to the point. It is a very calculating sort of handwriting."

Roe said it was different from the more obviously passionate writing of someone such as Mahler. "Rachmaninov is a more cool customer," he said. "And you can tell that from his own performances, which tend to be quite restrained compared to the flamboyance of the music."

What happened to the score after 1908 is not fully known although it was not in the suitcase the Bolsheviks allowed him to have when he left Petrograd for Helsinki after the 1917 revolution.

There is every chance that the sale, on 20 May, could bring an auction record. The last comparable sale was in 1994 when Schumann's autograph manuscript for symphony number 2 in C Major sold for £1.48m at Sotheby's.

Sotheby's said the manuscript was not studied in any detail when it was in the British Library. "It still holds its mysteries," said Roe, and is the only surviving manuscript providing any insight into the genesis and evolution of the 2nd symphony.