"BREAKING: Belmarsh Prison refuses permission for Julian Assange to participate in European Parliaments Sakharov Prize Award today," WikiLeaks tweeted.
The company also said that Assange's spouse Stella would attend the event on her husband's behalf on Tuesday.
Julian Assange was one of the three finalists shortlisted for the award in October.
Assange was apprehended in London in 2019 on a US arrest warrant and faces extradition to the United States and a sentence of up to 175 years in a maximum security prison if convicted of espionage.
The 51-year-old Australian, wanted by the US government on charges of disclosing classified information and violating the Espionage Act, took refuge at the Ecuadorian Embassy in June 2012 until his arrest in April 2019, after which his extradition hearings began.