The London Philharmonic Orchestra has suspended four musicians for nine months for using its name when they called unsuccessfully for the cancellation of a concert by an Israeli orchestra at the Proms.

The move follows the indefinite suspension of an unnamed LPO violinist after she allegedly launched an anti-Israel "rant" when Israeli musicians appeared at the Royal College of Music before the concert at the Royal Albert Hall earlier this month.

In a statement, Tim Walker, the LPO's chief executive, and Martin Hohmann, its chairman, said the suspensions sent "a strong and clear message that their actions will not be tolerated … the orchestra would never restrict the right of its players to express themselves freely, however such expression has to be independent of the LPO itself.

"The company has no wish to end the careers of four talented musicians but … for the LPO, music and politics do not mix."

They added that the orchestra had no political or religious affiliations and strongly believed in the power of music to bring peace and harmony to the world, not war, terror and discord.

The LPO suspended cellist Sue Sutherley and violinists Tom Eisner, Nancy Elan and Sarah Streatfeild until June 2012 after they signed a letter as members of the LPO denouncing the Israel Philharmonic Orchestra (IPO) as an instrument of the country's propaganda.

It said: "Denials of human rights and violations of international law are hidden behind a cultural smokescreen. The IPO is perhaps Israel's prime asset in this campaign … Israel's policy towards the Palestinians fits the UN definition of apartheid."

Other signatories of the letter, which appeared in the Independent newspaper two days before the concert, included: the composer Raymond Deane; violinists Catherine Ford and Roy Mowatt (of the Orchestra of the Age of Enlightenment); violinist Susie Meszaros (of the Chilingirian Quartet); and 16 other musicians.

The IPO's concert on 1 September was barracked by protesters so noisily that the BBC suspended its live broadcast, although the musicians completed their performance of works by Bruch, Webern, Albeniz and Rimsky-Korsakov, conducted by Zubin Mehta.

Sarah Colborne, director of the Palestine Solidarity Campaign, which organised protests against the concert, said: "Would the London Philharmonic Orchestra have punished musicians speaking out against apartheid South Africa, when a similar call for boycott was supported by artists, performers and sports people internationally?

"It is staggeringly bad judgment for the LPO to be seen to be attacking musicians who are simply voicing support for human rights and defending the civil right to call for a boycott of institutions which lend strategic support to Israel's occupation.

"If the LPO really wishes not to appear to be taking sides, and supporting an occupying nation against an occupied people, it must end the ridiculous suspension of these four musicians immediately."