An independent biomechanical analysis showed Shillingford's elbow extended beyond the permitted 15 degrees on his standard off-break delivery and his doosra, the ICC said in a statement.
"He is suspended from bowling in international cricket until such time that he has submitted to a fresh analysis, which concludes that he has remedied his bowling action," it said.
Shillingford's team-mate, Marlon Samuels, was subjected to similar tests, which showed his standard off-break was legal but his quicker delivery was not, meaning he can only bowl his slower ball in international matches.
The actions of the West Indies pair were reported during their team's second Test against India in Mumbai last month.
The ICC ordered them to undergo independent testing which was conducted in Australia while the players participated in the West Indies' first two Tests in New Zealand.
Both players have previously been banned from bowling because of illegal actions.
Samuels did not bowl for three years after first being reported in 2008, while Shillingford was banned in 2010 and returned a year later after undergoing remedial work on his action.
The ICC said if Samuels was reported again for a suspect action in the next two years and analysis backed up the concerns, he would face an automatic one-year ban.
There was no immediate reaction from the West Indies. The ICC said both players had the right to appeal.
The West Indies' third and final Test against New Zealand begins in Hamilton on Thursday. They trail the series one-nil.