The European Union came under heavy attack from the United States, Japan and other major powers Wednesday for missing the agreed deadline to submit an initial market access offer in the global talks to open up trade in basic telecommunications services.

This is the second time the EU has missed a deadline for submission, prompting some countries, including the United States, Canada, Japan and Australia, to accuse Brussels of stalling tactics.Responding to the barrage of criticism, the EU delegation said Brussels is ''fully committed" to the process, and pledged to file its offer by Oct. 6.

The 15-nation EU missed the July 31 filing deadline, pleading technical difficulties, but promised to file its proposal in time for the round of negotiations opening Sept. 18.

The talks, held under the auspices of the World Trade Organization, must be concluded by April 30, 1996.

Forty-one nations, including all the major developed countries and leading developing nations such as Brazil, Argentina and India, are taking part in the talks.

Moreover, another 31 nations, including China, Poland, Russia, Indonesia, Malaysia, Pakistan and South Africa, are full observers in the sessions.

According to EU officials who spoke on condition of anonymity, this week's EU delay was triggered by objections to the European Commission's draft offer raised by Spain, Portugal and Belgium.

Portugal and Spain want different language in parts of the text, while internal problems are behind Belgium's reaction.

The commission, which had said it wants to put forward a strong offer, feels the Iberian countries' changes would weaken its draft offer.

The dispute is likely to send the issue to the EU's influential 113 Committee meeting next week, the source said. The committee, which reviews key EU policies, is made up of members of the commission and EU member governments.

EU's delay aside, bilateral negotiation sessions, which have been

discussing reports from a number of delegations, are reported to have made solid progress.

The Slovak Republic was the eighth nation to come forward and submit a

draft offer, and the Venezuelan delegation said it hoped to make an offer by October 17.

Reflecting the boost in momentum, the U.S. side said that by the end of this week it planned to hold 28 bilateral negotiating sessions. Similarly, Canada said it planned to hold 20 similar bilateral meetings and Australia 15.

The U.S. offer provides for an almost unlimited access of foreign telecom companies to the nation's market to countries providing a similar market opening. The only limitations are those on the ownership of radio licenses and restrictions imposed by individual states.

In a thinly veiled warning to other countries, the U.S. delegation said Washington was not looking simply for a "standstill" that would effectively bind current access levels, but was looking for firm and real liberalization commitments. A U.S. official who asked not to be named, singled out Japan as offering too little by simply proposing to maintain the status quo.

Australia, Canada, Mexico, New Zealand, and Hong Kong are also among the countries that have proposed their market-opening offers.

Australia stated its offer was comprehensive and provided genuine market access and national treatment commitments.

U.S. and Western trade officials said they expect serious negotiations to start Oct. 6 when chief negotiators and senior telecom officials meet here in a session scheduled to coincide with the International Telcom 95 exhibition.