The Japanese and British companies that have been buying most of Iran's oil exports stopped their purchases yesterday, refusing to pay the latest $2.50-a-barrel price increase.

The Royal Dutch Shell group is expected to follow suit today.

That the companies were willing to say no on economic grounds was a measure of how plentiful world oil supplies are at the moment, oil analysts said. Together the companies have been buying about 800,000 barrels a day.

Iran announced it was placing an embargo on sales to Japan after the Japanese trading companies refused to pay $35 a barrel retroactive to April 1. But "embargo" seemed a strane word to use when a buyer just chose not to buy because of price.

Would-be buyers can go next door to Iraq and get up to 400,000 barrels of oil a day, probably for about $5 a barrel less than Iran was demanding. Oman already has agreed to sell the Japanese more oil.

"There's plenty of oil around, one place or another," one Carter administration official said, and most industry analysts agreed.

Iraqi production has dropped from 3.7 million barrels a day last year to no more than 3.3 million, analysts noted. Nigeria has had to cut output from about 2.4 million barrels daily to about 2.2 million, and Venezuela's production has fallen by a similar amount.

All of that 800,000 in currently unproduced oil should be available to the companies that would not pay the Iranians' price.

State Department spokesman Hodding Carter praised the Japanese refusal to pay, calling the Iranian price increase "highway robbery. The decision that they took on economic grounds . . . not to submit to those prices was in fact a very courageous step."

And he added, "It is one which we greatly appreciate and one which we hope others will emulate."

Administration energy officials said Japan -- which has been getting about 500,000 barrels of Iranian oil daily, about 10 percent of its supply -- would seek to replace it with lower-cost crude from other countries. Only if Japan is unsuccessful would it likely turn to the International Energy Agency oil-sharing agreement under which the United States and other industrial nations might share their supplies with Japan.

Earlier this month, Sir David Steel, chairman of British Petroleum, said the loss of Iranian oil to his company would be "minimal." BP has been getting about 125,000 barrels a day from Iran.

An Analyst with the Petroleum Industry Research Foundation, Gary Ross, said that unless the loss of sales by Iran were to trigger some unexpected response by some other oil producer, "the world can get by without the Iranian oil."

Ross said his organization has estimated that for 1980 as a whole only about 28 million barrels a day of production will be needed from nations in the Organization of Petroleum Exporting Countries. Even with a drop of 800,000 barrels a day in Iran's output, OPEC still will be producing more than that.

Other analysts pointed out that, at $35 a barrel, the oil would cost more than the value of the products that could be refined from it. Moreover, spot-market cargoes of crude were available in Europe for about $33, less than the Iranians wanted for oil put into a tanker in the Persian Gulf.

U.S. officials expect Iran to try to sell its oil elsewhere, perhaps to Third World nations at a price less than they were asking from the Japanese and European companies. "It's not at all clear that all this oil will be lost to the market," one officials said.

However, the development makes continuation of a high level of production in Saudi Arabia more necessary for maintaining stability in the market, the official cautioned. The Saudia are producing 9.5 million barrels a day and would prefer to pump no more than 8.5 million.