Dear Daniel Barenboim,

Your last performance in London left me sleepless, and not in a good way. I was sorry to miss your new piano and the Schubert recitals, being out of town, but I watched on video your Edward W. Said lecture and it left me troubled and regretful, both at the wretched state of the world and at the irrational direction your inventive mind was taking.

We have talked, you and I, in years past about your symbiotic friendship with Said, a para-Palestinian US-based academic who stirred you to engage with the Middle East conflict.

The results have been wholly beneficial. You formed a West-Eastern Diwan Orchestra with young musicians from both sides of the divide, attenuating prejudices and accentuating dialogue. In a maestro milieu where most music directors look no further than the fourth horn, you stuck your neck out for a prickly cause that was always bound to earn you more heartache than acclaim.

I applauded your gumption, your physical courage and your personal commitment to Israel, where you continue to maintain a home. You talked to me once, in Paris half a lifetime ago, of your idyllic boyhood in the young state of Israel, where girls wore no lipstick and men no ties. Something of that naked idealism, that transcendence of spiritual values over material display, is imprinted on the artist you became - one of the foremost classical interpreters of our time, perhaps the only one who commands the attention of world leaders when you speak on great issues of the day.

So when you deliver the Said lecture it has resonance, the more so in a tilted series where past speakers have been determinedly one-sided - Ahdaf Soueif, Noam Chomsky, Raja Shehadeh. Said himself, as you well know, was a visceral opponent of the Oslo Agreement, an Egyptian of American parentage who was Palestinian only by accident of his birth in Jerusalem. You were always waging an unequal fight against stacked, sometimes falsified, odds.

Your lecture was uncontentious, an appeal for more investment in music education. Nothing you hadn't said before. Where the session came to life was when an audience member asked if you supported BDS - the Palestinian-led campaign for boycott, divestment and sanctions of Israel. Daniel, this was your answer:

"I think the boycott movement BDS is absolutely correct, it's perfectly right and necessary with one limitation, one criticism - that it refuses any contact with anything that has to do with Israel..."

You went on to hedge and haver a bit, concluding that "It is very short-term and not positive for any future for the Palestinians to boycott anything that has anything to do with Israel. Anything to do with government policies, yes."

Let's just parse that sentiment. It is perfectly right and necessary - your words, Daniel - for the world to isolate, anathematise and subject the state of Israel to economic siege, so long as it is directed at government policies and does not discriminate against actively dissenting Israelis. In other words, for an Israeli Jew to be fit for human intercourse in 2015, he or she must first declare, like conversos in Catholic Spain, ''I am not one of them''.

Daniel, what you and I think of the present Israeli government is neither here nor there. It is the only government currently available in the state of Israel. By boycotting the government, you are stifling the state in which your parents chose, freely and with the best motives, to bring you up. "Yet each man kills the thing he loves..."

What are the grounds for boycott? The BDS states: "For decades, Israel has denied Palestinians their fundamental rights of freedom, equality, and self-determination through ethnic cleansing, colonisation, racial discrimination, and military occupation." There is no contesting the ugly truth of much of that statement.

It rings, however, much uglier and much truer when applied Russia in Chechniya and Crimea, Turkey in Cyprus and France in central Africa. So why are sanctions perfectly right and necessary against Israel, Daniel, when there are so many bigger offenders? You have no qualms in conducting annually in Turkey, where the state president, Erdogan, called only last week for the "liberation of Jerusalem" from the infidel Jews. Turkey is a flagship supporter of BDS.

Contrary to your caveats, BDS regularly disrupt concerts by freethinking Israeli artists in London and elsewhere. Their most recent target were the Jerusalem Quartet who were playing in the selfsame South Bank hall where you embraced the BDS campaign. The philistines are upon you, Daniel.

By endorsing BDS, you parted company with many who love the things you love and share your peaceful aspirations. I lost sleep because I saw you, Daniel, sleepwalking into a moral and intellectual abyss. There may be no turning back.

With deep regret,

Norman Lebrecht