The Norwegian mass murderer Anders Breivik has written to a German neo-Nazi terror suspect, urging her to use her forthcoming trial for alleged complicity in racist killings to promote their far-right ideology.

Breivik, 33, was jailed in the summer for detonating a bomb in Olso in July 2011 and shooting dead scores of young Labour Party members at a political summer camp because they held multicultural views. His attacks claimed 77 lives. Breivik, who claims he was fighting a far-right anti-immigrant “crusade”, wrote to the suspected German terrorist Beate Zschäpe in May while she was being held for questioning in a Cologne prison, Der Spiegel magazine revealed.

Zschäpe, who is charged with complicity in the murders of a policewoman and nine immigrant street vendors, is due to stand trial later this year. Addressing her as “dear sister Beate” in his letter, Breivik urged her to use her case to canvas her far-right “political motives”.

“If you make it clear that you really are a militant nationalist, then you will be seen as a courageous heroine of national resistance who has done and sacrificed everything to stop multiculturalism and the Islamification of Germany,” he told her.

Breivik also went out of his way to applaud the murders carried out by the National Socialist Underground (NSU), the terror cell Zschäpe belonged to. However, he claimed his own attack on Norway’s “political elite” was more effective than terrorising “minorities” such as immigrants.

Breivik’s closing remark gave a disturbing insight into his fanatical state of mind. “We are both among the first raindrops presaging the powerful and cleansing storm which is heading towards Europe,” he wrote. “We are martyrs of the conservative revolution and we should be extremely proud of our sacrifices and our efforts.”

Germany’s state prosecutor’s office said Zschäpe never received the letter because judges ordered it to be confiscated. It was not immediately clear how the letter managed to pass unnoticed out of the Norwegian prison Breivik was being held in when he wrote it.

Zschäpe is believed to be the only surviving member of the NSU, which idolised Hitler and wanted to up a “Fourth Reich” in Germany. Its reign of terror came to an end when police found the bodies of its two ringleaders – Uwe Mundlos and Uwe Böhnhardt – in a burnt-out caravan in eastern Germany last November. Investigators established that they had committed suicide after robbing a bank. Zschäpe was caught and arrested.