Global 24-hour-a-day stock trading will move a step closer to reality April 22 when the U.S. over-the-counter market and the London Stock Exchange begin swapping electronic quotations on 600 stocks of international interest.

The information exchange is part of a growing movement within the world securities industry to create a global stock market that would be open almost 24 hours a day.

The new international link is being forged between the National Association of Securities Dealers, which operates the burgeoning over-the-counter, or Nasdaq, market, and the London Stock Exchange, which has developed an electronic trading system similar to that used by Nasdaq.

Within two years, NASD officials expect that the electronic link will allow traders on both sides of the Atlantic to initiate, negotiate and clear their trades using computers.

Until then, traders watching overseas price changes on their computer screens will negotiate trades on the transatlantic telephone.

The worldwide trading of securities is well under way. Trading already follows the sun from Tokyo to London to New York. Three New York firms recently won seats on the Tokyo Stock Exchange.

The New York Stock Exchange recently extended its morning hours so it could overlap with the London Stock Exchange, and the two exchanges are trying to set up a new electronic trading link that initially would trade several of the most heavily traded stocks.

In forming its link with London, the NASD continues its competitive battle with the NYSE, which has been holding talks with London officials for months.

Gordon S. Macklin, president of NASD, said he could not conceive of a gobal trading system that did not include the NYSE, but noted: "The winners in the battle for market share will be those that provide the highest degree of liquidity on the most economical terms."

An NYSE official said their new trading link would put them far ahead of the Nasdaq information-swapping arrangement because their system will allow actual trading using the computer link.

The success of international electronic trading will depend heavily on the service to be offered by the newly formed International Securities Clearing Corp., a subsidiary of the National Securities Clearing Corp. of New York.

A clearing house is used to settle trades between brokerage firms and handle the accounting and record keeping.

Macklin said he could not estimate how much trading would be generated by the new link.

The Nasdaq link offers Nasdaq and London a chance to extend trading hours. The London quotes will be available to NASD from 3 a.m. to 12 p.m. Washington time (8:30 a.m. to 5 p.m. in London).

But Nasdaq traders won't see the London quotes until the Nasdaq system opens at 8:30 a.m. Washington time. However, Nasdaq may decide to open its system earlier.

NASD will send its stock prices to London from 8:30 a.m. to 4 p.m. Washington time (1:30 p.m. to 9 p.m. in London.) The London exchange computer system will allow London traders to see Nasdaq quotes into the evening.

Nasdaq stocks to be displayed in London include those listed on the Nasdaq 100 and Nasdaq Financial 100 lists.

The first group includes the 100 largest nonfinancial stocks on the Nasdaq market, domestic and foreign. The second includes the 100 largest domestic financial companies. Then Nasdaq will add 100 of its non-British foreign stocks, traded in this country as American Depository Receipts.

The London exchange will furnish Nasdaq users in the United States with the Financial Times/Stock Index 100, called "Footsie," which includes the 100 largest companies in the British market. Another 200 London listings included in the system will be securities of companies with headquarters outside of the United Kingdom.

Nasdaq prices will be displayed in dollars; the London prices will be shown in the currency in which the stocks usually trade.

In time, the number of stocks displayed may be expanded to 500 from each market, Nasdaq said.

Formation of the new overseas link will involve a cooperative effort to share information needed by the SEC and London authorities for surveillance and investigation, Nasdaq recently told the Securities and Exchange Commission.