The number of Jewish settlers in the Israeli-occupied West Bank has doubled in the past two years, and while the pace of settlement will slow because of economic constraints later in the decade it will not fundamentally alter Israel's iron grip on the territory, according to a leading Israeli expert on the subject.

The population figures for the West Bank are contained in the latest report of the West Bank Data Base Project, an independent Jerusalem-based organization that monitors the size of the Jewish presence in the territory.

According to the report, at the end of 1984, there were 42,600 Jewish settlers living in 114 communities in the West Bank, compared to the 1982 population figures of 20,600 Jewish residents and 71 settlements. The growth rate was 34 percent in 1983 and 54.5 percent in 1984, most of it concentrated in large, well-established suburban settlements within commuting distance of Tel Aviv or Jerusalem, according to the report.

Building starts in the West Bank settlements have plunged to half the 1982 rate because of Israel's deteriorating economy, but the report contends that there is enough construction already under way to maintain the population growth rate of the last two years until 1987-88.

"We find, therefore, no reason to modify our projection that by the end of the decade the number of Israeli settlers will reach 100,000 unless major political changes occur," the study said.

In an interview, Meron Benvenisti, a former deputy mayor of Jerusalem and the director of the West Bank Data Base Project, said that the population trend figures indicate that "the political partition of Palestine today is, unfortunately, not possible." The West Bank, he said, "is not two areas that can be redivided" along the lines envisaged by most Middle East peace proposals, including President Reagan's 1982 peace initiative.

Benvenisti, a strong critic of Israel's settlement policy, has been making this point for years. It has provoked the sharpest criticism from Israeli liberals in the Labor Party, whose West Bank policy of "territorial compromise" involves returning portions of the West Bank to Jordan.

Benvenisti released his latest population figures at a time of stepped-up Middle East diplomatic activity, including the meeting in Washington this month between King Fahd of Saudi Arabia and President Reagan and a reported agreement on peace negotiations between King Hussein of Jordan and Palestine Liberation Organization Chairman Yasser Arafat.

"People are trying very hard, struggling with new peace formulas, but you can say the whole process will end up nowhere," Benvenisti said.

According to Benvenisti, government decisions to establish more or fewer new settlements will not alter the demographic trend toward an increasing Jewish presence in the West Bank.

Similarly, he argues that a "freeze" on settlements, which was advocated by Reagan when he made his peace initiative, would not be meaningful if it applies only to the establishment of new settlements.

"Now, to freeze settlements is to freeze construction on a particular street in an already established, prospering community," he said. "It is much more difficult to interfere in the open market process than it is to stop building new settlements."

Benvenisti said the government could slow the process by changing the West Bank's development status. Under existing Israeli law, the territory is included in Israel's highest priority development category, meaning that home buyers and developers are provided a number of financial inducements to invest in the territory. But to change this classification, Benvenisti said, "would be a real political statement, and even the Labor Party is not ready for that."