Israel's new law effectively banning political boycotts is unconstitutional and does grievous harm to freedom of expression and protest, three dozen eminent Israeli law professors have said in a petition.

The move followed prime minister Binyamin Netanyahu's robust defence of the law in the Knesset (parliament) on Wednesday in which he said he was "against boycotts aimed at the Jewish state".

The petition, sent to attorney-general Yehuda Weinstein, was signed by the deans of many of Israel's law schools, including some associated with the political right.

"This law is a classic case of the tyranny of the majority," said Alon Harel of Hebrew University, one of the instigators of the petition. "The majority aims at silencing, persecuting and threatening the minority. It conflicts directly with the principles established in Israel in the 1990s that entrench the right to freedom of speech in the legal system. It is the most cherished right in the Israeli legal system."

Under the Law for Prevention of Damage to the State of Israel through Boycott, an individual or organisation proposing a boycott may be sued for compensation by any individual or institution facing possible damage as a result. Evidence of actual damage will not be required.

It bans consumer boycotts of goods and services produced in West Bank settlements and the blacklisting of cultural and academic institutions in settlements. It also bars the government from doing business with companies that comply with boycotts.

Boycotts were a standard form of protest in Israel, Harel said. But the new law was a "non-neutral restriction".

"Speech or action which promotes one viewpoint is protected and sanctioned, yet speech which promotes another viewpoint is prohibited," he said.

Boycotts by ultra-orthodox Jews against the Israeli national airline El Al over flying on the sabbath or by Israeli tourists against Turkey following last year's flotilla had not been targeted, he said.

Harel said the new law had to be seen within a wider context: "Basically, Israel is still a lively democracy. But this is part of a campaign to win the political struggle not through free elections and political discourse but through silencing certain sections of society."

Several civil rights groups have launched a challenge to the new law in Israel's supreme court and high court of justice.

Another bill is to be brought before the Knesset next week which allows the investigation of the funding of human and civil rights groups in Israel. Many groups say this is unnecessary as their funding is totally transparent and they claim it is part of a wider campaign of harassment and an attempt to restrict their actions.

Two rightwing members of the Knesset announced on Wednesday they would present a further bill allowing the Knesset to veto supreme court appointments. The right has criticised its judges for decisions it considers to be against Israel's interests.

The bill, which is not widely supported, is unlikely to succeed. The speaker of the Knesset, Reuven Rivlin, said: "The threat to the supreme court is a danger to democracy."

Despite being absent for Monday night's vote in favour of the law, Netanyahu told the Knesset: "I don't want anyone to be confused. I approved the law. If I hadn't backed it, it wouldn't have passed. I am against boycotts aimed at the Jewish state."

He denied the new law damaged Israel's image. "What mars its image are the reckless, irresponsible attacks against the legitimate attempt by a democracy on the defensive to draw a line between what is acceptable and what isn't acceptable," he said.

Matthew Gould, the British ambassador to Israel, came under fire for saying in an interview with Israeli newspaper Maariv that the UK was concerned about the law.

"For a foreign diplomat to take such a public stance is highly unusual," a foreign ministry official said. "It is not customary for an ambassador to speak out against a legislative process."

In a separate development, nursery schools in Israel are to be required to raise the Israeli flag and sing the national anthem at least once a week to strengthen children's Zionist values. Kindergartens in Arab areas will be exempt from the requirement, issued by the education ministry.