Khodorkovsky, the former head of the Yukos oil giant, and his former associate, Platon Lebedev, went on trial on March 3, 2009, on charges of embezzling millions of tons of oil and money laundering.
The verdict was expected this week but was postponed at the last moment until December 27.
Both men have pleaded not guilty to embezzlement. They are currently in jail on fraud and tax-evasion charges that critics say were fabricated to punish Khodorkovsky for financing opposition parties.
The authors of the letter say the decision to postpone the reading of the verdict "proves that the verdict was not decided in the courtroom."
Prominent St. Petersburg human rights defenders Nina Katerli, Yevgeny Ikhlov, and Boris Vishnevsky are among the signatories.
Prominent writer Boris Strugatsky, who also signed the letter, told RFE/RL the decision to start reading the verdict on December 27 is an attempt by the authorities to deflect international interest in the case.
Strugatsky said all international journalists covering the trial will most likely go home to celebrate the holidays, and that that is exactly what the authorities want.
He added that Prime Minister Vladimir Putin's public statement on December 16 constituted "unacceptable pressure upon the court."
During a live TV question-and-answer session with the public, Putin called Khodorkovsky a thief and said that like any thief, Khodorkovsky should be in jail.
Read in Russian here