JERUSALEM - Israelis are looking fearfully beyond the end of Egyptian President Hosni Mubarak's rule, expecting it will force them to stiffen security across an extensive southwestern border and perhaps reoccupy a strategic corridor between Egypt and the Gaza Strip.

In the long term, it may require Israel to expand its military force and budget if a new Egyptian government comes under the sway of the Islamist Muslim Brotherhood, or otherwise casts into doubt the long-standing peace accord between the two nations.

Israel has relied for three decades on the assumption that it would never again fight a land war against the Arab world's most populous state, or worry about Egypt openly supporting militants in the Gaza Strip or elsewhere.

Many Israelis are now convinced that the Islamist group will play a greater role in whatever government emerges in Egypt, and that their country needs to prepare for anything from a cutoff of natural gas shipments from Egypt to increased arms smuggling into the Gaza Strip. The Muslim Brotherhood opposes Egypt's peace treaty with Israel and is the inspiration for the Palestinian group Hamas, which controls Gaza and rejects Israel's existence.

Even before Mubarak's regime began to crumble, Israelis complained about weapons being smuggled across the poorly secured eight-mile-long Egypt-Gaza border. Now, security experts said it was likely that if Mubarak falls, Israel may have to retake control of what is known as the Philadelphia Corridor, from which Israel withdrew in 2005.

In December, Israel approved construction of a fence along its southwestern border with Egypt to block infiltrators, mostly African laborers seeking work. Former military and intelligence officials said overall border security measures in that area would probably need to be expanded.

Sixty-five percent of Israelis said Mubarak's fall would be bad for Israel, according to a poll published in the daily newspaper Yedioth Ahronoth on Thursday. Only 11 percent thought the results would be positive for Israel. Likewise, a majority - 59 percent - said a Muslim regime was more likely to emerge in Egypt, compared with only 21 percent who said the government would remain secular.

Israeli defense experts are already discussing whether the army is large enough to confront what could become a new threat.

"For more than two decades, the structure of the Israeli forces was based on the assumption that Egypt is in no way a potential threat to Israel,'' said Ephraim Sneh, a former Israeli deputy defense minister. "I don't say that today Egypt is a threat. But what's happening there may change the status of Egypt vis-a-vis Israel as far as strategic balance is concerned. And that's why Israel will have to change the structure of all of its forces."

After Israel signed the 1979 peace treaty with Egypt, its defense budget dropped from 30 percent of its gross national product to 7 percent, according to Giora Eiland, a former Israeli national security adviser and former head of the army's planning branch.

While Eiland saw no doubt Israel would expand its defense budget as a result of developments in Egypt, he said the change in leadership could also impact Israel's ability to act regionally.

"In the past 30 years, Israel could take almost for granted that no matter what happens between Israel and the Palestinians, Israel and Hezbollah, Israel and Syria, Egypt would not act militarily'' against Israel, Eiland said. Israel was able as a result "to take some risks'' in terms of the size of its force, he added.

The Israel Defense Forces does not make public the size of its force. But according to Tel Aviv University's Institute for National Security Studies, an independent research institution, Israel maintains a standing army of 176,500 soldiers and 445,000 reservists.