The British Library is appealing to the public to help it raise the money to buy an archive of original musical scores that includes the manuscript for Beethoven's Ninth Symphony.
The library needs to raise Ј200,000 more to meet the Ј1m asking figure for the Royal Philharmonic Society's collection.
The society's collection includes working papers, minute books and correspondence with great composers and performers.
It contains 250 manuscript scores, and took 250 years to build up.
In a letter written by Beethoven just eight days before his death, in 1827, the composer pledges to write what would have been his 10th symphony in the society's honour.
"I will undertake to return to the society my warmest thanks by engaging to compose for it a new symphony, sketches for which are already in my desk," Beethoven wrote.
Now, however, the society is unable to meet the expense of curating its archive.
A spokeswoman told BBC News Online: "We have absolutely no resources."
The collection had been offered to the British Library, rather than to any other institution, she said, because so that it would remain accessible to the public, and in London.
"Nothing can replace the magic of seeing and working from an original musical score"
"It's an important scholarly resource, and the library can preserve it for the future," she said.
The library is currently exhibiting some of the items, including the Beethoven score and the score for Mendelssohn's first symphony.
It has until the end of the year to raise the money.
It says it is confident of doing so - but warns that if it does not, the archive could end up going abroad and being broken up.
The money raised so far is a mix of the British Library's own funds, grants and cash pledges.
Chief among the pledges is a Heritage Lottery Fund commitment to contribute Ј400,000 - but before the promise is realised, the total must be met.
Launching the library's appeal, renowned pianist Mitsuko Uchida, said: "Nothing can replace the magic of seeing and working from an original musical score."
To work from the original score was to "follow the composer's thoughts and corrections across the page", she said.