Here is a small selection of events that have taken place in Britain since the end of Israel's Operation Cast Lead in Gaza earlier this year.

The government has imposed a partial arms embargo on Israel and failed to vote against the Goldstone report in the U.N . The charities War on Want and Amnesty International U.K. have both promoted a book by the anti-Israeli firebrand Ben White, tellingly called "Israeli Apartheid: A Beginner's Guide." The Trades Union Congress at its annual conference has called for boycotts of Israeli products as well as a total arms embargo.

In the media, the Guardian newspaper has stepped up its already obsessive campaign against the Jewish state to the extent that the paper's flagship Comment is Free Web site frequently features two anti-Israeli polemics on one and the same day. The BBC continues to use its enormous influence over British public opinion to whitewash anti-Semitism and Holocaust denial in the Middle East. Its Web site, for example, features a profile of Hamas that makes no mention of the group's virulent hatred of Jews or its adherence to a "Protocols of Zion"-style belief in world-wide Jewish conspiracies.

Readers may be surprised to learn, therefore, that the British media and political establishment is apparently cowering under the sway of a secretive cabal of Zionist lobbyists who have all but extinguished critical opinions of Israel from the public domain.

Such charges have been aired to mass critical acclaim this week in a landmark  documentary, "Inside Britain's Israel Lobby," on Channel 4—the same outlet that offered Iran's Holocaust-denying president, Mahmoud Ahmadinejad, an uninterrupted, seven-minute propaganda slot on Christmas Day last year.

The makers of the documentary—top Daily Mail columnist Peter Oborne and TV journalist James Jones—have also written about their program in the Guardian. Both furiously deny that they are peddling conspiracy theories. But the mindset we are dealing with was neatly exposed by the authors' own explanation on how their suspicions were aroused that something sinister is at work in the corridors of British power.

It all transpired, they told readers ominously, during an address earlier this year by Conservative Party leader David Cameron at a dinner hosted by the Conservative Friends of Israel.

"The dominant event of the previous 12 months had been the Israeli invasion of Gaza," they wrote. "We were shocked Cameron made no reference in his speech to the massive destruction it caused, or the 1,370 deaths that resulted, or for that matter the invasion itself. Indeed, our likely future prime minister went out of his way to praise Israel because it 'strives to protect innocent life.' This remark was not intended satirically."

Since it is inconceivable, the authors obviously believe, that anyone could honestly credit Israel with anything other than the most damnable motives it must therefore follow that those who do in fact praise the Jewish state must be being paid or bullied into doing so.

If you think this all sounds familiar, you'd be right. Messrs. Oborne and Jones produced an extensive pamphlet accompanying the documentary, which openly claimed inspiration from none other than John Mearsheimer and Stephen Walt, authors of "The Israel Lobby and U.S. Foreign Policy"—another conspiracy theory alleging malign Zionist influence in the United States.

But if Messrs. Mearsheimer and Walt at least felt the need to dress up their polemic in pseudo-academic wrapping paper, the sheer amateurishness of the British documentary they inspired is breathtaking. There was the endless superimposition of the Israeli Star of David on to the British flag, which, along with some absurdly melancholic background music, was presumably designed to prepare viewers for an astonishing series of revelations. But of course such revelations actually never materialized.

It turns out from the documentary itself that the allegedly secretive Jewish donors have been quite open in declaring their interests in accordance with the law. One of them, Poju Zabludowicz, the billionaire funder of the Britain Israel Communications and Research Centre (BICOM) is good friends with Madonna—not exactly the kind of company you'd choose if you were trying to hide behind a veil of obscurity.

Much is also made of the influence of Friends of Israel groupings in the British Parliament. Such allegations are, of course, rendered ridiculous with a moment's reflection on the countervailing influence of vast amounts of Arab oil money, not to mention the fact that membership in such groups for many parliamentarians is either purely formal or outright meaningless. Michael Ancram, for example, a former Northern Ireland minister and a member of Conservative Friends of Israel for more than 30 years, is famous for calling for talks with Hamas.

Given the paucity of the arguments, it would be tempting to dismiss the whole thing as unimportant. Would that we could. The documentary has already provoked a torrent of abuse against British Jews, not least on Channel 4's widely read Web site, whose moderators have seen fit to approve dozens of postings about the Zionist lobby's "seditious behavior," its "disgusting attack on British democracy," "the hand of global Zionism at work," and several along the lines of the following, which said flatly: "We want our country back. The agents of a foreign power embedded at all levels of our government and politics need flushing out."

If this sort of language takes hold, a bad situation in Britain may be about to get a whole lot worse.

Jewish leadership organizations have long feared accusations of divided loyalty between Britain and Israel and, ironically given the charges now being made against them, are frequently criticized in their own communities for failing to be sufficiently robust in Israel's defense. The risk is that some may now be panicked into silence.

Non-Jews who call for a more reasoned discussion of Israel—already a small and diminishing group in Britain—will likely face additional slanders against their integrity: Since there is supposedly no reasonable case to be made in favor of the Jewish state, we must have sold out to the "Lobby."

Such calumnies cannot be allowed to stand. Now more than ever, the forces of reason and decency must continue the fight to be heard.

Mr. Shepherd is director of International Affairs at the Henry Jackson Society. His new book, "A State Beyond the Pale: Europe's Problem With Israel," has just been published by Weidenfeld and Nicolson.