Wall Street banks will have three months to convince regulators to weaken a proposed ban on their ability to trade for their own profit, after officials on Tuesday put the Volcker rule out for comment.

The rule is aimed at avoiding a repeat of the 2007-2009 financial crisis by curbing excessive risk-taking, but it has been difficult for the government to craft a rule that protects the trades Wall Street does on behalf of clients.

Regulators are giving the public until January 13, 2012, to comment on the rule required by last year's Dodd-Frank financial oversight law.

That is more time than expected, and could result in more pressure to change elements of the rule.

The proposal includes more than 350 questions that regulators want interested parties to weigh in on.

The Federal Reserve and other bank regulators acknowledged in the proposal that it will be challenging for the government to identify proprietary trading that will be banned under the rule.

The proposal said drawing the line between prohibited and permitted trading often involves subtle distinctions that are difficult both to describe comprehensively within regulation and to evaluate in practice.

The rule is expected to have the most impact on large banks such as Goldman Sachs, Morgan Stanley and JPMorgan Chase, and could shave off billions of dollars in annual revenues.

The Volcker rule, named after former Federal Reserve Chairman Paul Volcker, aims to prevent banks from making risky trades by prohibiting short-term trading for their own profit in securities, derivatives and other financial products.

It will also prohibit banks from investing in, or sponsoring, hedge funds or private equity funds.

The idea behind the rule is to prevent banks that enjoy some sort of government safety net, such as deposit insurance on customer accounts or access to Fed money, from using that backstop to make money for themselves.

Wall Street has pushed back against the rule. Industry groups have argued that it should include broad exemptions for trades done to make markets for customers, and for those trades used to hedge against certain risks in the banks' portfolios.

The proposal includes both types of exemptions, but it is difficult to determine how they will work in practice.

At a minimum, the proposed rule would increase costs and discourage firms from making markets in securities, said Dwight Smith, a partner at Morrison & Foerster LLP, a law firm which works with investment banks.

It calls for some very precise management of that business and some very detailed record-keeping, said Smith. It becomes very cumbersome.

The impact of the proposed rule will likely be discussed with investors as banks host quarterly earnings calls starting Thursday with JPMorgan.

Barclays Capital analyst Jason Goldberg said in an October 7 research report that executives would be well-served to show investors they will cope with the Volcker rule restrictions, with bank stocks already beaten down this year.

Regulators on Tuesday acknowledged that controversy has surrounded the Volcker rule from the outset.

The proposed rule has been noted as long, the issues are complex, so I think we made the right decision in allowing the full 90 days for comment, said John Walsh, acting director of the Office of the Comptroller of the Currency, which oversees the nation's largest banks.

Walsh spoke at a meeting of the Federal Deposit Insurance Corp board which agreed to put the proposal out for comment.

The Securities and Exchange Commission is due to discuss the Volcker rule proposal at a meeting on Wednesday. The Commodity Futures Trading Commission has yet to announce how it will proceed.

The Volcker rule proposal released by bank regulators on Tuesday is largely similar to a draft of the rule leaked last week that received a mixed reaction from industry groups.

The Securities Industry and Financial Markets Association, for instance, raised concerns about whether the exemption for trades intended to make markets for customers is too narrow.

A note released on Monday by Bernstein Research said, based on the draft, that the rule will have a very negative impact on the business models of fixed-income trading for Wall Street brokers. Bernstein estimates the impact could be 25 percent less in revenues.

To stop foreign-owned banks reaping a windfall or skirting the rule, prohibited trades cannot be conducted if a party to the transaction is a U.S. resident or a bank employee involved in the transaction is physically located in the United States.

Under the Dodd-Frank law, the Volcker rule goes into effect on July 21, 2012.

(Reporting by Dave Clarke and Alexandra Alper in Washington and David Henry in New York; Editing by Andrea Ricci and Tim Dobbyn)