The world-renowned scientist has accused Conservative minister Jeremy Hunt of "cherry picking" evidence to support the changes and says the service is at risk.
Hunt rejected Hawking's charges today, but the public spat underscored the strains on the health service after years of cost-cutting.
Hawking, a supporter of the opposition Labour Party, was diagnosed with motor neurone disease in 1962. He says he "would not be here today if it were not for the service."