Mr Haw, 62, set up a camp in London's Parliament Square in 2001 in protest against UK and US foreign policy.
In March, a court order forced him to move his camp on to the pavement.
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Hide AdA statement on his website, from his family, said: "As you know, he was battling lung cancer, and having treatment in Germany. He left us in his sleep and in no pain, after a long, hard fight."
Fellow members of the Parliament Square Peace Campaign said the authorities "should forever be ashamed of their disgraceful behaviour towards Brian".
Later this year, Westminster Council is set for a court bid to get the camp moved off the pavement opposite the Houses of Parliament, which could see it removed permanently.
Mr Haw began his round-the-clock protest against the UK's policy in Iraq and elsewhere on 2 June 2001. It began as a response to sanctions and British and US raids on Iraq, but the scope widened after the 9/11 attacks on New York and Washington DC, and the invasions of Afghanistan and Iraq that followed.