Regulations aimed at reducing the risk of another financial crisis are starting to upend a key part of the bond market that expedites trading in everything from Treasuries to junk bonds.

The U.S. repurchase, or repo, market where banks and investors borrow and lend Treasuries and other fixed-income securities shrunk to $4.6 trillion daily outstanding last month, down 35 percent from a peak of $7.02 trillion in the first quarter of 2008, based on Federal Reserve data compiled from its 21 primary dealers.

From fewer repos to lower inventories of bonds, financial institutions are responding to more stringent capital standards imposed by regulators around the world. Already, the group of dealers and investors that advise the U.S. Treasury say that they see declines in liquidity in times of market stress, including wider gaps between bid and offer prices and the speed of completing trades. The potential consequences are higher borrowing costs for governments, companies, and consumers.