NEWARK, N.J. (AP) _ Western Union Corp. said Wednesday that the final piece to its complex restructuring plan has been approved by debtholders, rescuing the company from the brink of bankruptcy proceedings.

By Wednesday morning, the required two-thirds of the company's debtholders agreed to exchange their debentures for stock in a restructured company, a spokesman said.

The new company will be formed by the merger of Western Union and its chief subsidiary, Western Union Telegraph Co., and the purchase of ITT Corp.'s international telex service.

All the other elements of the company's drawn-out reorganization plan now will fall into place, including raising $500 million through the sale of high- yeild, high-risk ''junk bonds'' and the purchase of ITT World Communications Inc., said spokesman Warren Bechtel.

All elements of the plan were mutually dependent on each other, and the failure of one would have scrapped the whole project.

Western Union, based in Upper Saddle River, announced on Dec. 16 it had reached an agreement with its financial adviser, Drexel Burnham Lambert Inc., to underwrite the bonds.

Western Union also will be infused with $25 million from a group led by financer Bennett S. LeBow of New York, whose specialty is investing in troubled companies. LeBow would take a controlling interest in the company.

''We're certainly pleased that the process was brought to a successful conclusion,'' Bechtel said.

Western Union officials have said that without approval of the plan, the company would have to file for bankruptcy protection.

The 136-year-old company that linked 19th-century United States with telegraph wires has been in trouble since 1984, when banks cut off a $100 million line of credit and the company faced a cash crisis.

Analysts reacted cautiously to the news of the restructuring plan. The dilution of stock will keep the company profitless for at least 18 months, they said.

''It's certainly a welcome alternative to bankruptcy,'' said John Niebuhr, vice president of Moody's Investors Service. ''It's going to be tough sledding for a couple of years.''

''How fast they will turn around seems somewhat problematical,'' added Fulton S. Holmes, vice president of utility research at Thomson McKinnon Securities Inc.

Holmes expressed pessimism about the telex portion of the telecommunications business, and characterized the overall plan as a ''reluctant, arm-twisting kind of agreement.''

Bechtel declined comment on Holmes' statement.

Niebuhr and analyst Geoff Johnson of Argus Research pointed positively to the acquisition of ITT's telex service.

Johnson said the ITT purchase adds about $100 million to Western Union - $60 million in cash flow and $40 million in reduced expenses.

''It's a huge boost,'' Johnson said. ''As far as the common stock goes, it's not going to be positive. Profitability is two years away.''

Already this year, Western Union has cut the massive losses of the previous few years.

The company lost $59 million in 1983, $58 million in 1984, $367 million in 1985 and $531 million last year. Western Union has reported $17.2 million lost for the first nine months of 1987.