A statement by his family on brianhaw.tv, said: "It is with deepest regret that I inform you that our father, Brian, passed away this morning.
"As you know he was battling lung cancer, and was having treatment in Germany.
"He left us in his sleep and in no pain, after a long, hard fight."
Mr Haw, who was 62, had been battling for his right to remain living in a tent in Parliament Square.
He had been stationed in the square for 10 years, and had fought off a series of legal objections to his presence there.
The latest saw the Greater London Authority get him and his supporters thrown off the grass area at the centre of the square.
Later this year Westminster Council will attempt to get the camp moved off the pavement, which could see it removed permanently.
Mr Haw, began his round-the-clock protest opposite the Houses of Parliament against the UK's policy in Iraq and elsewhere on June 2, 2001.
It began as an angry response to economic sanctions and British and American bombing 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.
Mr Haw's tent and ragtag collection of horrific pictures of war victims and hand-written posters with slogans like "baby killers" was a familiar sight in the square.
Civil rights campaigners got behind him as he saw off various attempts to force him to move.
In November 2004, ministers announced provisions in the Serious Organised Crime and Police Bill that could have seen him removed from Parliament Square.
Initially, the High Court ruled Mr Haw's protest was not covered by the Act because it started before the new law came into effect.
But the case was taken to the Court of Appeal which, in May 2006, ruled he would have to apply to the police under the Act for permission to continue his protest.
Fellow members of the Parliament Square Peace Campaign said the authorities "should forever be ashamed of their disgraceful behaviour towards Brian".
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