Up to one in five people in this country is a Holocaust denier. That has to change.

Holocaust Memorial Day, which the UK and many other countries have just marked, ought to be a moment of national and global solidarity with the Jewish people. Alas, the truth is rather different. Anti-Semitism is gaining ground, especially on the Left, while the isolation and demonisation of Israel gains official recognition, not least here in Europe. One example: the Irish Parliament, the Dail, is now debating whether to ban the sale and import of all goods from the West Bank. If passed, the new law would single out Israel with a boycott enforced by penalties up to five years in prison.

Here in Britain, many Jews fear a Labour government led by Jeremy Corbyn, who has promised to recognise Palestinian statehood on his first day in office. Despite many promises by the Labour leader to distance himself and his party from anti-Semitism, he has singularly failed to do so.

The problem, though, is bigger than Corbyn. A poll commissioned by a government charity, the Holocaust Memorial Day Trust, makes for sobering reading. One in twenty British adults, 5 per cent, thinks that the Holocaust never happened, 8 per cent think the scale has been “exaggerated” and 19 per cent think fewer than two million Jews were murdered. These attitudes, including “gross minimisation of the number of victims”, correspond to the working definition of Holocaust denial given by the International Holocaust Remembrance Alliance, a widely respected body to which the UK belongs.

This means that up to one in five people in this country is a Holocaust denier. This is not as incredible a statistic as it seems, because ignorance is rife. A majority of us, 64 per cent, simply do not know how many Jews were killed by the Nazis or grossly underestimate the number. This is a lamentable failure of education by successive governments. But it also reflects the ubiquity of such vile propaganda.

One example must suffice: The Times today reports that YouTube has been running anti-Semitic advertisements from a group, Deir Yassin Remembered, that defends Holocaust denial. This is the group whose UK branch is led by Paul Eisen, who has written that he accepts the label of Holocaust denier and who has appeared at events alongside Jeremy Corbyn, though the Labour leader now claims that he has no contact with Eisen. The Times reports that these advertisements, which a spokesperson of the Board of Deputies of British Jews says are clearly anti-Semitic, would have been viewed and approved as “appropriate” by staff at Google, YouTube’s parent company. The paper says that even after being approached, YouTube still has these videos on its platform, though it is no longer running them as ads.

The public is thus exposed to a wave of propaganda that, inter alia, equates the actions of Israel to those of Nazi Germany and denies the legitimacy of the Jewish state. Given the scale and power of social media, the insidious influence of such lies has a toxic impact on attitudes to Jews and Israel. The mutations of this mendacity take innumerable forms, especially in the Muslim world, where there is little or nothing in the public sphere to counteract the view that Israel is illegitimate and iniquitous. Being Muslim does not make anyone anti-Semitic, of course, but so much anti-Semitic material comes from Islamist sources that young Muslims are especially at risk of this kind of indoctrination.

Last week Liam Fox, the International Trade Secretary, signed a free trade deal with his Israeli counterpart. Anglo-Israeli trade now exceeds £10 billion per annum, making the Jewish state one of Britain’s largest trading partners, thanks especially to the technology sector, in which Israel is a world leader. In the Middle East, only Saudi Arabia does more business with the UK — all the more remarkable, given that Israel has no oil. Israel is also a crucial partner in intelligence and counterterrorism.

It is therefore a matter of self-interest, as well as common decency, for the Government to do what it can to stop the minds of our young people, a rising proportion of whom are Muslims, from being polluted by anti-Semitic propaganda, whether online or elsewhere. Theresa May, whose personal philosemitism is not in doubt, should strengthen and enforce laws against the propagation of anti-Semitism and Holocaust denial. Social media platforms need to know that when the British say “Never Again”, we really mean it.