MADRID — The former treasurer of Spain’s governing Popular Party, Luis Bárcenas, who has been at the heart of a widening corruption investigation, was being held on Friday after a judge decided that he presented a flight risk.

The judge, Pablo Ruz, who issued the order late Thursday, said he took the step in part because of recent findings suggesting that Mr. Bárcenas held money in other countries, including the United States and Uruguay, in accounts that have not been frozen. Mr. Bárcenas, who is in a jail outside Madrid, is to be held without bail until a possible trial, for which no date has been set. His lawyers said they would appeal.

The denial of bail was one of the most dramatic developments to date in the case against Mr. Bárcenas, who is being investigated on suspicion of tax fraud and other financial crimes, including running a slush fund that made regular payments to Prime Minister Mariano Rajoy and other senior party officials.

After he resigned as treasurer in 2009, Mr. Bárcenas is believed to have left his office with nine boxes of documents. Asked on Friday whether he feared possible blackmail from his former treasurer, Mr. Rajoy said, “Neither now nor at any moment in the past has this occurred.” He called on the judiciary to handle the case “with celerity.”