Wall Street is growing increasingly worried about incoming European rules restricting research payments, with a group of US research analysts penning a letter to EU policymakers and US asset managers planning to pare back the number of brokers they use if the rules go through.

Under the review of the Markets in Financial Instruments Directive, EU regulators are proposing to separate research costs from the fees paid by portfolio managers to brokers for executing trades, in a process known as "unbundling".