InTrade has been shut down, pending an investigation of its practices.
The online betting site announced the move in a statement, citing "circumstances recently discovered that may include financial irregularities."
Users will not be able to withdraw money from their accounts.
Ireland-based Intrade allows users to place bets on future events by buying and selling contracts in a similar way to how financial markets operate.
Users can bet on everything from who will win the 2016 presidential election, to who be the next Pope to how much snow will fall in a given winter.
It is favored by academics and journalists who often use the site's odds as indications of public sentiment, particularly when it comes to elections.
"With sincere regret we must inform you that due to circumstances recently discovered we must immediately cease trading activity," Intrade said.
"These circumstances may include financial irregularities. During the upcoming weeks, we will investigate and determine the necessary course of action."
The company did not elaborate, and its future is uncertain.
In November Intrade suffered a severe blow to its business when regulators forced it to suspend all trading in the United States - its biggest market.
The US Commodity Futures Trading Commission (CFTC) filed a lawsuit against the company, founded in 1999, arguing that it violated US trading laws.