TEL AVIV — There is a social time bomb ticking away in Israel.

With each closure Israel imposed on the West Bank and Gaza Strip in the wake of repeated terror attacks, thousands of Palestinian workers were prevented from working in Israel. Previously such workers held primarily low-paying agricultural and construction jobs.

To make up for the shortfall in unskilled laborers, the Jewish state began authorizing the import of workers from farther afield — Thailand, Romania, Turkey, the Philippines.

The number of such workers now stands at about 250,000 — of whom some 150,000 are working illegally, according to government officials.

Many of the illegal workers originally came to Israel as tourists and decided to stay. Others came to Israel under valid work permits and outstayed the one-year legal time limit.

Their wages, far below those of the average Israeli worker, are nonetheless higher than they would earn at home.

So they remain in Israel, often living in cramped, substandard housing.

They are not a burden to social services, so long as they have valid visas, which are issued by the Interior Ministry only after an applicant has received a work permit from the Labor Ministry.

But once the permits expire, the workers cannot pay their health bills. Their families are not entitled to the social benefits given to Israelis, including education.

Nor do the illegal workers have much hope for any improvement of their lot. They can become Israeli citizens only if they marry Israelis. If they have children while in Israel, their offspring remain illegal aliens.

President Ezer Weizman recently warned of the dangers of hosting a large foreign community without proper facilities.

Many construction workers, for example, live in improvised housing provided by their employers. There are often as many as eight people to a room, and they live in slum conditions.

The Interior Ministry has warned that if illegal workers are deprived of medical attention they may become a source of serious diseases that could spread throughout the country.

Although police officials say the crime rate among illegal workers is no higher than in Israeli society at large, some feel that poverty and substandard living conditions will in time ratchet up the crime rate among them.

In an effort to clamp down on illegal workers, a Labor Ministry inspection team daily raids construction sites to check foreign workers' papers. But the team numbers only 70 officials — hardly a sufficient force to cope with a quarter-million foreign workers.

The scope of the problem has recently come under the scrutiny of the Interior Ministry.

Some 40,000 illegal workers come from the former Communist bloc, according to ministry figures.

Another 45,000 are from Asia and Africa, 15,000 from Latin America and 10,000 from Arab countries such as Lebanon, Jordan, Egypt and Morocco.

Most of the construction workers are Romanians and Turks; most of the farm workers Thais; Filipinos are very popular as nannies or as companions for the elderly; and most of the Africans do cleaning jobs.

With an unemployment rate of 6 percent, the Israeli economy can well tolerate the presence of legally imported foreign workers.

As a result of the influx of the foreign workers, the look and feel of many Israeli neighborhoods are changing. An international gallery of people, mostly from Ghana and Nigeria, can be seen in the area around southern Tel Aviv's former central bus station.

A large Tel Aviv café called Turkish Coffee hosts scores of Turks who play backgammon and cards after work, or watch Turkish television and drink tea.

At Herzliya Pituah, the luxury quarter of Herzliya that is home to diplomats and Israeli high society, Filipina nannies take children out for strolls or take the boss' car for a shopping errand.

"The Israelis like us; they trust us. Why should they replace us?" said Simo Delfin, 46, of the Philippines.

Delfin, who came to Israel eight years ago after divorcing his wife, left six children in Manila.

He cleans homes and offices in Herzliya and Tel Aviv, makes at least $2,000 a month and sends $800 home to his children.

But he dares not visit them, fearing that Israeli authorities will not allow him back in. Originally allowed into Israel on a tourist visa, he is now illegally employed in the Jewish state.

The Interior Ministry recently announced that it was seeking to remove 100,000 foreign workers. These deportations, which can be contested in court by workers or their employers, may reflect a broader political motive.

The government of Prime Minister Benjamin Netanyahu has partially lifted the closure on the West Bank and Gaza in order to ease the prolonged financial hardship it has caused to Palestinian workers.

The closure was imposed after the first of four Hamas terror attacks on Israel in late February and early March.

As part of the relaxed closure, an additional 10,000 Palestinian workers were issued work permits earlier this month, bringing the total number of Palestinians in the West Bank and Gaza Strip allowed to work in Israel to 35,000.

Ten years ago, some 160,000 Palestinians held jobs in Israel.

Closures are regularly imposed by Israel in response to terror, but they have brought repeated criticisms from the international community, which points to a markedly decaying Palestinian economy.

And some Israeli officials, while not attacking the policy of temporary closures, have recently argued against those seeking to establish a permanent separation between the Israeli and Palestinian populations as a way of coping with terror attacks.

The separation plan, these officials say, would only serve to give Palestinians an additional argument for the formation of an independent Palestinian state that would have borders drawn roughly along the same lines that are being established, de facto, by the repeated closures.

If the closures are ever fully lifted, many foreign workers in Israel would have to be displaced to make room for their Palestinian counterparts.

Moreover, authorities cannot tolerate a situation of "soft borders" where illegal workers insinuate themselves into the fabric of Israeli society.

Given these concerns, the issue of foreign workers — both legal and illegal — may well start moving to a position of greater priority on the agenda of the Netanyahu government.