Hartley was sent off for allegedly calling referee Wayne Barnes a "f****** cheat" just before half-time in Leicester's 37-17 victory at Twickenham.
The Rugby Football Union confirmed the suspension rules Hartley out of the British and Irish Lions tour of Australia, with Ireland's Rory Best now favourite for a call-up.
Hartley has the right of appeal.
Hartley pleaded not guilty at the hearing, which was held at the Epsom Downs Holiday Inn.
The England international argued after the match, via Northampton director of rugby Jim Mallinder, that his comments had been aimed at Leicester hooker Tom Youngs after a scrum had collapsed and the Tigers had been awarded a penalty.
Two minutes before that incident, Barnes had warned Hartley he would take action if the player made any further comments the official felt were directed at him.
The three-man disciplinary panel, chaired by Judge Jeff Blackett, found Hartley guilty of a mid-range offence. He is free to play again on September 1.
The suspension covers the Lions tour and three pre-season friendlies for Northampton.
Judge Blackett said: "We cannot get around the fact that Wayne Barnes was certain that Mr Hartley's comments were directed at him.
"Wayne Barnes said that if he had had any doubt in his mind, then he would have given the player the benefit of the doubt.
"In terms of sanction, calling a referee a cheat is an attack on his integrity and contrary to the core values of rugby and therefore we have taken a serious view and provided a serious sanction."
The Lions issued a statement which said: "Warren Gatland and his fellow coaches will now consider all their options before making any further announcement."
Hartley has won 47 caps for England and he captained his adopted country against South Africa last summer but his career has been chequered by disciplinary incidents.
In 2007, Hartley was hit with a 26-week ban after being found guilty of eye-gouging Wasps forwards James Haskell and Jonny O'Connor in an Aviva Premiership match.
In March 2012, Hartley was banned for eight weeks for biting Ireland flanker Stephen Ferris in a Six Nations match. In December he was suspended for two weeks for punching Best in a Heineken Cup match.