Ken Livingstone was caught off guard recently. In an interview with SomethingJewish.co.uk he was asked how London was going to celebrate next year's 350th anniversary of the readmission of the Jews to England. He replied: "Well, until you mentioned it, I hadn't remembered when we had re-admitted the Jews. I can't even remember who threw them out!" Fortunately for the mayor, his office is one step ahead of him. "Jews were welcomed by Cromwell in 1656," reads a new guide to Jewish cultural events in London. "Throughout 2006, we pay special tribute to their heritage, presence and future in Britain."

The celebrations kick off next week, when two public conferences will debate the question "Why Did Cromwell Take Us Back?". The Arts Council has funded a new play; the Greater London authority is considering an autumn event in Trafalgar Square, complete with giant menorah; and Tate Modern plans to host an anniversary concert.

The trouble is, Cromwell did not readmit the Jews to England in 1656. In fact, he didn't readmit them at all. Certainly they'd been expelled, by Edward I in 1290. In 1655, Menasseh ben Israel, a leader of the Jewish community in Amsterdam, came to England to persuade Cromwell to let them back in. In December that year, a conference of lawyers, merchants and theologians met to discuss the issue, but failed to reach a verdict. According to one account, "the Preachers kept pelting one another so furiously with Texts of Scripture, and wiping their Spectacles, that the Debates continued for four Days; though very little was said on the Question of the Jews' resettlement."

Members of the existing Jewish community in London (yes, there were Jews in Britain before 1656, albeit in hiding) then petitioned Cromwell to let them worship openly, but their petition was ignored. A merchant named Thomas Violet reflected the views of the majority when he declared, in 1661, that "it is Felony for any Jew to be found in England". In the centuries that followed, Jews were repeatedly threatened with harassment and expulsion, and continued to be classed as aliens.

The non-event only became fact in the late 19th century, when the mass immigration of Jews from Eastern Europe prompted widespread antisemitism. The existing Jewish community was keen to emphasise the positive aspects of immigration, and they made use of an ideal precedent. At the same time, the reputation of Cromwell was being transformed from that of radical villain to liberal hero and so the first Resettlement Day was held in 1894.

Despite the historical reality, the story of Cromwell and the Jews is told in the new Dictionary of National Biography, on undergraduate history courses, and in speeches by the chief rabbi. It's not hard to understand why minority history is often celebratory. But whose interests, ultimately, do these myths serve? As the government's official guide for immigrants makes clear, Britain regards itself as an exceptionally tolerant country, and the readmission story contributes to that self-image. Unfortunately, for those wanting proof that religious integration is possible, however, they hould avoid citing the events of 1656.