Last Friday, I found myself at a drinks reception for journalists at the Royal Albert Hall following Verdi's Requiem at the BBC Proms. It was a very civilised occasion and everybody agreed that this year's season had been a great success. I had very much enjoyed the evening's performance, which included an impressive contribution from the BBC Proms Youth Choir. It was the moment to celebrate the healthy state of classical music in the UK and recognise that it was safe in the hands of the next generation. It was the last place I had expected to encounter antisemitism.

But as I entered the room, two friends approached me in a state of genuine shock having just been part of a heated exchange. Another journalist had challenged a BBC freelancer about her contribution to a programme about the Holocaust and had told her to "get back in the oven".

One of my friends was shaking with fury after telling the individual that such comments were completely unacceptable and that he should leave immediately.

It turned out that there had been some history between the two people involved in the exchange and they had argued about Brexit on a previous occasion. I decided to ask the man what had possessed him to use such an appalling antisemitic insult. He said it was meant as a joke, but he was upset because the woman had said "f*** you" after discovering he had voted to leave the EU. I told him there was no moral equivalence between the use of a common profanity and suggesting that someone should have died in a Nazi death camp. I also told him he should apologise and leave.

The incident has troubled me ever since. Should the man have been forcibly removed from the Royal Albert Hall, or taught a lesson with fisticuffs in the street? Or was it enough that several people in the room expressed their disgust and he was humiliated in front of his peers? I don't know. But what I do know is that something has happened over the past months in this country to give a licence to the open expression of antisemitic views. In my complacency, I never expected to hear someone tell another person to "get back in the oven" at a Radio 3 drinks reception, let alone at a concert of music by the man who wrote that great anthem of liberation. Va Pensiero, the Chorus of the Hebrew Slaves. And yet I did.

She'd been told to 'get back in the oven'

I had a further encounter with casual antisemitism at the weekend when I took to Twitter to defend Michael Foster, the Labour donor suspended from the party for describing the people around Jeremy Corbyn as stormtroopers.

To me, it seemed to send out all the wrong signals to eject a prominent Jewish businessman just when the party was trying to escape the furore surrounding the Chakrabarti Report. "Wrong," said a Twitter Corbynista by the name of Cool Daddy. "Foster is the epitome of the negative caracature (sic) of Jews. He ENCOURAGES antisemitism by his own actions." Cool Daddy was joined by the delightfully named Smeggypants who added: "Removal of the vile Michael Foster from Labour was superb. Corbyn should remove the rest of the Zionist 5th column." Just two tweets, I know, but it seemed to me that I was witnessing a lesson in classic antisemitic rhetoric being played out in these 280 characters. Here we had the familiar trope of the sinister Jewish businessman being held to blame for the prejudice that swirled around him due to very fact of being rich, successful, influential and, crucially, Jewish. And, in the second example, Foster becomes that other stock bogeyman figure: the treacherous Jew, who can't be trusted because his true loyalty lies with Israel.

It may be tempting to blame this bubbling up of the most ancient hatred on the Brexiteers or Momentum, but this doesn't fully explain it.

On Saturday night, I sat down with the family to watch the Last Night of the Proms on TV and felt reassured. A Finnish conductor with a wry turn of phrase, a Peruvian tenor in an Inca costume, flags from around the world, including a smattering of EU starred banners. Yet, through the fug of tolerant, eccentric, harmless patriotism, I couldn't stop thinking about what had happened the night before.

For now, I will consider it a timely wake-up call rather than a requiem for decent, liberal values.