The GMC also found that Wakefield failed to disclose he was paid in conjunction with the lawsuit, or that he had a patent related to a new MMR vaccine in development when he submitted the 12-child case report to be published in a scientific journal.
Wakefield declined an interview with ABCNews.com, but issued a statement saying, "The allegations against me and against my colleagues are both unfounded and unjust and I invite anyone to examine the contents of these proceedings and come to their own conclusion."
Wakefield was found "proven" to have committed the ethical breaches by the GMC Fitness to Practice Panel, but his time before the government is not over yet.
On April 7, the GMC is scheduled to decide whether these breaches constitute "serious professional misconduct" and if so, how Wakefield will be reprimanded or whether he will lose his license.
Wakefield and his former colleagues may see their fate hanging on the April decision, but the public may not agree.
Scientists who say Wakefield's hypothesis has been discredited long ago with large studies finding no link between vaccines and autism view the ethical breaches as an interesting, but meaningless, side point.
Similarly, the parent groups who stood behind him in rallies and in press statements say his theories have led to anecdotally successful treatment in their children and also doubt that a finding by the GMC will change any minds.
When the public got word of Wakefield's work, worried parents skipped vaccines, and the percentage of children who were not vaccinated in the United States rose from 0.77 percent in 1997 to 2.1 percent in 2000, according to an article by Dr. Michael Smith in the journal Pediatrics.
A similar drop-off in vaccinations occurred in Britain, weakening overall immunity and putting those who are too young to get vaccinated at risk for measles, mumps or rubella.
Although the U.S. Centers for Disease Control declared the United States cleared of measles in 2000, fewer vaccinations brought back the disease in a 2008 outbreak.
At least 131 cases were reported to the CDC, and 11 percent of the cases were hospitalized. A handful of children in Britain died of the measles around the time of the U.S. outbreak.
Offit said that scientists didn't sit idly by after the news broke in 1998.
"People spent millions and millions of dollar looking at this hypothesis that he raised," he said.
But according to Offit -- and international studies supported by the CDC as well as a 2004 review of large international studies by the Institute of Medicine -- high-quality studies could not confirm Wakefield's hypothesis about vaccines.
Although Wakefield's original hypothesis did not mention mercury, a row also arose over whether a mercury-based preservative called thimerosal found in other vaccines cause autism. As a precaution, California Gov. Arnold Schwarzenegger and several other governors banned mercury from childhood vaccines in 2004. But autism rates have only increased since then.
"He had his day in scientific court, this is his day in ethics court," said Offit.
Robert Field, an expert in health care ethics and a professor of law and public health Drexel University in Philadelphia, agreed that the charges against Wakefield posed ethical issues.