Irish online gambling business InTrade said it was suspending its operations and withholding payments to members while it is under investigation.
"At this time, and until further notice, it is not possible to make any payments to members in accordance with their settled account balance until the investigations have concluded," the company said in an online posting.
InTrade said it would immediately stop operations on the website and "settle all open positions and calculate the settled account value of all member accounts."
It also said it would "cease all banking transactions for all existing company accounts immediately."
The posting did not give a reason for the investigation, or say if it was in-house or by a regulatory agency.
"These circumstances require immediate further investigation and may include financial irregularities," the company said, noting that it was shutting down "in accordance with Irish law."
The website manages non-sporting-event gambling, including bets on the outcome of political elections, the weather and financial benchmarks, such as how high a stock index might go by the end of the day.