In 1968, in full knowledge of the outrage it would cause, Enoch Powell, a defeated candidate for the Tory leadership, warned of a race war to come unless non-White immigration to Britain was halted. Powell, hitherto admired for his formidable independent intellect, cited the fears of a constituent that “one day the Black man will hold the whip hand over the White”. The political establishment shuddered in disbelief — and fear that Powell’s message would prove popular.

The so-called Rivers of Blood speech was the last time in Britain that a mainstream politician played the race card so nakedly. But will it be the last? Some tribunes of Nigel Farage’s right-wing populist party, Reform UK, and even a few ambitious Tories, are flirting with a political language that has not been heard in polite society for half a century.

Not everyone remembers how close Powell’s gambit came to success, but Farage, a youthful admirer of Powell who once chauffeured his political hero to a Conservative constituency meeting, will be an exception.

Powell was sacked within 24 hours as an opposition spokesman by Tory leader Edward Heath. London’s dock-workers and market porters, however, marched in his support; in one Gallup opinion poll, three quarters of voters said they agreed with Powell’s sentiments, while 69 per cent said Heath was wrong to have banished him to the back benches. Nonetheless, he believed that victory would ultimately be his — another election defeat for Heath would deliver him his party’s leadership. Only the Tories’ wholly unexpected win against Labour in 1970 proved him wrong.

The country has changed for the better since then: just 2 per cent of Britons feel uncomfortable about the idea of living next door to someone of a different race, down from 10 per cent in 1981, according to a global study conducted by King’s College, London, two years ago. On that score, Britain is the most tolerant country on Earth, bar Sweden and Brazil.

But ambitious politicians can tap into fears about unrestricted mass migration and the country’s ability to integrate arrivals from different cultures. The United States may have been founded as a migrant melting pot, but in Britain the proportion of those born overseas in the population historically has been low — at least until recently. At the General Election last year, two thirds of those who voted Conservative or Reform said they were “uncomfortable” with hearing foreign languages in public, according to a new survey by the National Centre for Social Research.

Relaxation of controls by Labour and Tory governments — culminating in Boris Johnson’s post-Brexit administration presiding over nearly a million new arrivals in a single year — has stoked fears about the country’s cohesion and its ability to absorb large numbers from different cultures.

Last month, Reform’s newest Member of Parliament, Sarah Pochin, made headlines when she declared that the disproportionate number of Black and Asian actors in TV adverts “drives me mad”. Farage called her comments “wrong and ugly”, but not racist in intent. Pochin apologised, saying her words were “phrased poorly”, but her outburst was not an isolated one. Careless talk about racial differences is becoming more widespread, and not just in Reform.

A rising Tory star, shadow home affairs minister Katie Lam, let fly in a newspaper interview that large numbers of people with legal status in Britain should have their right to stay withdrawn and “need to go home… what that will leave is a mostly but not entirely culturally coherent group of people”. Lam was dressed down by Conservative headquarters for “misunderstanding” the party’s (admittedly opaque) migration policy. Robert Jenrick, who appears to be running a permanent undeclared campaign to be Conservative leader, also complained recently about visiting an area of Birmingham where he “didn’t see another White face” and said it wasn’t the kind of country he wanted to live in. The shadow justice secretary later qualified his remarks — it wasn’t about “the colour of your skin or your faith”, but about wanting people to live alongside each other, he said.

The pair’s remarks prompted Tory party leader Kemi Badenoch to warn shadow ministers not to stray from her “core messages”, calling the breakdown in discipline “not helpful” during a private meeting with her frontbenchers last week, The Sunday Times reported.

Older hands who remember Powell and were happy to see the back of him are alarmed. In an interview with The Times, Michael Heseltine, a nonagenarian veteran of both Heath and Margaret Thatcher’s cabinets, denounced Farage as “a fascist”. The use of the “f” word is inflammatory, but there is no mistaking that the temperature is rising.

An event I attended last month registered the howling winds of change. The Spectator magazine’s annual parliamentary awards at the Raffles Hotel saw the whole Westminster bubble released from its usual restraint — the modern equivalent of a festival of fools for politicians. The most memorable acceptance speech of the night touched on race. Home secretary Shabana Mahmood, nominated “Minister to Watch”, a hardliner on immigration of Pakistani descent and Muslim faith, had words for Pochin: “Sarah, if you are here tonight, I am sorry you have to watch another Brown face up on this stage.”

There are dangers in opening the Overton window on race. Reform faces a critical choice — does the populist party play by the old rules of civility, or does it follow the path taken half a century ago by Powell? And how far will the Tories go in their pursuit of the same voters? It’s all very well apologising for remarks and claiming to move on, but the impression left on a non-White audience is ugly. Having extended family who are dark-skinned, I am forced to ask how they must feel if this political language becomes the norm. It’s morally wrong — and responsible political leaders should nip it in the bud.

• Martin Ivens is the editor of The Times Literary Supplement. Previously, he was editor of The Sunday Times of London and its chief political commentator