MOSCOW, NOV. 26 -- Soviet leader Mikhail Gorbachev is prepared to extend his stay in the United States by one or two days if the Dec. 7-10 summit nears a breakthrough on the next step in arms control -- a treaty cutting strategic nuclear arsenals by 50 percent, a senior Soviet official said today.

Georgi Arbatov, the director of the Institute of the United States and Canada of the Academy of Sciences, told a news conference that Gorbachev's schedule allowed for an extension of the summit if a breakthrough was at hand.

"Comrade Gorbachev is not able to engage in tourist programs," said Arbatov, an adviser to Gorbachev and one of the Soviet Union's leading experts on North America. But if "it should turn out that one day would be needed to reach an agreement on cutting strategic offensive arms by 50 percent, I would say that comrade Gorbachev would stay there for an extra day or even two to achieve this result," Arbatov said.

During their last summit in Reykjavik, Iceland, in October 1986, the two leaders extended their talks by several hours in the hope of agreeing on a treaty on medium- and shorter-range nuclear weapons. After more than a year of intensive negotiations in Geneva, that agreement has now been concluded and its signing will be the centerpiece of the Washington summit.

Prime Minister Margaret Thatcher told Parliament that Gorbachev had accepted an invitation to stop for several hours in Britain on his way to Washington for the summit in two weeks.

Western diplomats have said Gorbachev may have come under pressure from his wife, Raisa, to spend more time in the United States to meet the American people. At one time U.S. and Soviet officials discussed a possible extended visit that would have included more travel within the United States.

In California, where President Reagan was vacationing, White House spokesman B.J. Cooper said Gorbachev would be welcome to stay longer. "It's been up to them all along as to how long he would stay," he said.

The Communist Party daily newspaper Pravda said today the Soviet Union expects next month's Washington summit to produce "concrete results" on the next phase of nuclear disarmament -- a 50 percent reduction in long-range strategic nuclear arsenals. Reagan has agreed in principle to travel to Moscow in May or June next year to sign an accord providing for deep cuts in strategic weapons. Officials from both sides have said it is possible to reach an agreement by then.

Arbatov said the Soviet side considered progress towards a strategic arms agreement "a top priority" of the Washington meeting.

In an editorial focusing on the Washington summit, Pravda said the Kremlin has clearly outlined its position on disarmament, and the success or failure of the meeting lies with how the United States approaches the summit.

"Our country approaches the summit in Washington with a firm intention to press for . . . concrete results in the cause of removing the nuclear threat. Life itself demands a businesslike and constructive search for mutually acceptable solutions on the entire range of problems," Pravda said.

If the Senate ratifies the medium-range accord formally called the Intermediate-range Nuclear Forces treaty, the Kremlin has said all medium- and shorter-range missiles will be destroyed within three years.

Arbatov said it was risky to predict whether the Senate would ratify the accord.