His statement followed newspaper reports describing how his 29-year-old wife, Victoria, suffers from an eating disorder and is attending a clinic for treatment.
Lord Spencer said that his wife, a former model, was suffering from "psychological problems" but attacked the "hypocritical and evil" tabloid media for harassing her at the clinic. He said that this had been "constant" over the last fortnight at the private Farm Place clinic in Ockley, Surrey.
He claimed that reporters had posed as medical correspondents, prospective patients and even close friends of Countess Spencer to attempt to gain access to her.
"If ever proof was needed that sections of the tabloid newspaper business in this country are riddled with hypocrisy and evil then it was provided today," he said of the story in the News of the World. "How the sad tale of a mother-of-four receiving treatment for psychological problems can be treated as news fit for public consumption must be beyond all reasonable people. To use someone's medical illness to fill your newspaper has to be the ultimate proof that sick minds dominate this part of the British media."
Lord Spencer said he planned to make a formal complaint to the Press Complaints Commission and would test if the watchdog was as "toothless" as critics have suggested. He said his wife needed "peace to address her problems without distraction and with considerable bravery".
Lord Wakeham, chairman of the commission, said he would take any complaint received from Lord Spencer "very seriously" and would be "investigated thoroughly". However, he refused to comment on the report.
A News of the World statement said: "No News of the World journalist or representative has at any time made any attempt to contact Countess Spencer at the Farm Place clinic. Nor has any News of the World journalist or representative used any form of subterfuge or deceit to gain access to her or to any other patient."