LONDON -- Paul McCartney has sent support to a Russian punk band whose members face prison for a stunt against President Vladimir Putin.

In a letter released Thursday, the former Beatle urged three members of all-female band Pussy Riot to "stay strong" and called on authorities to allow them freedom of speech.

McCartney said that "I and many others like me who believe in free speech will do everything in our power to support you and the idea of artistic freedom."

The band members have been in jail for more than five months because of an anti-Putin prank in Moscow's main cathedral, and face a maximum seven years in jail.

Celebrities including Madonna and Bjork have called for them to be freed.

A judge is due to rule on their case Friday.

Supporters Rally For Pussy Riot

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  • A security guard at the Christ the Saviour Cathedral, right, runs toward Pussy Riot supporters holding Cyrillic letters reading "Blessed are The Merciful" in Moscow, Wednesday, Aug. 15, 2012. (AP Photo/Novaya Gazeta, Yevgeny Feldman)

  • An unidentified supporter of Pussy Riot speaks to the assembled press outside the Christ the Saviour Cathedral, in Moscow, Wednesday, Aug. 15, 2012. (AP Photo/Alexander Zemlianichenko)

  • Mikhail Kriger

    Russian police officers detain opposition activist Mikhail Kriger during an unsanctioned opposition rally in Moscow, Russia, Friday, Aug. 10, 2012. (AP Photo/Mikhail Metzel)

  • Peaches

    Background actors wearing masks perform during the recording of a music video of Canadian musician and performance artist Peaches in support of members of the feminist punk group Pussy Riot in Berlin, Germany, Wednesday, Aug. 8, 2012. (AP Photo/dapd/Adam Berry)

  • Background actors show a balloon shaped as a mouth reading "Free Pussy Riot" during the recording of a music video of Canadian musician and performance artist Peaches in support of members of the feminist punk group Pussy Riot in Berlin, Germany, Wednesday, Aug. 8, 2012. (AP Photo/dapd/Adam Berry)

  • Masked female activists demonstrate in support of members of the feminist punk group Pussy Riot in front of the Russian Embassy in Berlin, Germany, Thursday Aug. 9, 2012. (AP Photo/dapd/Oliver Lang)

  • Peaches

    Canadian musician and performance artist Peaches, left, talks to actors wearing masks during the recording of a video of in support of members of the feminist punk group Pussy Riot in Berlin, Germany, Wednesday, Aug. 8, 2012. (AP Photo/dapd/Adam Berry)

  • Peaches

    Background actors wearing masks perform during the recording of a music video of Canadian musician and performance artist Peaches in support of members of the feminist punk group Pussy Riot in Berlin, Germany, Wednesday, Aug. 8, 2012. (AP Photo/dapd/Adam Berry)

  • Nadezhda Tolokonnikova, Maria Alekhina Yekaterina Samutsevich

    Feminist punk group Pussy Riot members, from left, Nadezhda Tolokonnikova, Maria Alekhina and Yekaterina Samutsevich sit in a glass cage at a court room in Moscow, Russia, Wednesday, Aug. 8, 2012. (AP Photo/Alexander Zemlianichenko)