The International Cricket Council (ICC) said on Monday that the 30-year-old right-armer's deliveries had been bowled while exceeding the allowed 15 degrees of tolerance.
He will now be suspended until he has submitted a fresh analysis which concludes he has remedied the problem.
Shillingford's West Indies team-mate, Marlon Samuels, has also been reprimanded by the ICC for an illegal action.
The body has deemed that Samuels' quicker deliveries exceed 15 degrees, meaning he can only continue to bowl off breaks until his quicker action is corrected.
The ICC said in its findings that the analyses of the pair had been performed at the University of Western Australia.
Shillingford and Samuels were reported for their actions after the second day of their Test with India last month. They both have previous suspensions on their record for illegal actions.