The petition creator, who is identified only as 'Damien', says on the petition site: "Give charities a break. The PM knows how valuable the work of charities is and the pressure they relieve on state services."
Helen Donoghue, director of the Charities' Tax Reform Group, said: "We are delighted that this has become a genuinely public issue, and it is gathering support hour by hour."
The petition can be signed at http://petitions.pm.gov.uk/Charities/. The deadline for signing is 21 June.