On 20 April 1968, the shadow defence secretary, Enoch Powell, delivered his notorious “rivers of blood” speech. The address represented a fierce attack on Commonwealth immigration and the Wilson government’s Race Relations Bill.

The bill, which entered into law as the Race Relations Act 1968, sought to criminalise the refusal of housing, employment, or public services to persons on the grounds of colour, race, or ethnic or national origin. It built on the Race Relations Act 1965, which only applied in “places of public resort”.

Powell stood in the 1965 Conservative leadership election, securing the support of only 15 MPs. Edward Heath, who was elected as Tory leader with the support of 150 MPs, appointed Powell as shadow defence secretary.

The “rivers of blood speech” language would see Powell widely condemned and cast out of frontline politics.

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Powell’s speech was delivered at the Conservative Political Centre in Birmingham.

The Tory MP began by claiming that the “supreme function of statesmanship is to provide against preventable evils”. But he said that such evils “are not demonstrable until they have occurred: at each stage in their onset there is room for doubt and for dispute whether they be real or imaginary.”

He also argued that discussion of these alleged threats are naturally unpopular because “people are disposed to mistake predicting troubles for causing troubles”.

Powell presented himself as someone willing to confront uncomfortable truths. Discussing future dangers, he said, was “the most unpopular and at the same time the most necessary occupation for the politician”.

To illustrate public anxiety about immigration, Powell recounted a conversation with a constituent.

The shadow defence secretary said this unnamed individual wanted to settle abroad because “in this country in 15 or 20 years’ time the black man will have the whip hand over the white man”.

Powell anticipated criticism for repeating such remarks, stating: “I can already hear the chorus of execration. How dare I say such a horrible thing? How dare I stir up trouble and inflame feelings by repeating such a conversation?”

But he claimed that “thousands and hundreds of thousands” of people were of the same opinion.

Powell argued that Britain was undergoing a “total transformation to which there is no parallel in a thousand years of English history.” on account of immigration. He suggested that there could soon be “three and a half million Commonwealth immigrants and their descendants”, rising to “five to seven million” by the end of the century.

He added: “Of course, it will not be evenly distributed from Margate to Aberystwyth and from Penzance to Aberdeen. Whole areas, towns and parts of towns across England will be occupied by sections of the immigrant and immigrant-descended population.”

Powell declared that Britain was “busily engaged in heaping up its own funeral pyre”.

He promoted a policy of “re-emigration”, which he argued could “appreciably alter the prospects.”

Powell then turned to the proposed Race Relations Bill and defended those who sought “to discriminate in the management of his own affairs between one fellow-citizen and another.”

He argued that this legislation was akin to “throwing a match on to gunpowder. The kindest thing that can be said about those who propose and support it is that they know not what they do.”

Powell claimed that many people were afraid to express these concerns publicly, warning that the “sense of being a persecuted minority” was growing among English people.

The most controversial section of the speech came at its conclusion.

Quoting a line from the Roman poet Virgil, Powell declared: “As I look ahead, I am filled with foreboding; like the Roman, I seem to see ‘the River Tiber foaming with much blood.’”

The Conservative MP added: “Only resolute and urgent action will avert it even now. Whether there will be the public will to demand and obtain that action, I do not know. All I know is that to see, and not to speak, would be the great betrayal.”

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The following day, 21 April, Heath sacked Powell from the shadow cabinet.

In a statement, Heath said: “I have told Mr Powell that I consider the speech he made in Birmingham yesterday to have been racialist in tone and liable to exacerbate racial tensions. This is unacceptable from one of the leaders of the Conservative Party.”

An editorial in the Times newspaper labelled Powell’s remarks “evil”.

It read: “This is the first time that a serious British politician has appealed to racial hatred in this direct way in our postwar history.”

Powell’s speech nonetheless attracted some public support.

On 23 April, 1,000 dockers marched on parliament to protest his sacking and the Race Relations Bill. The protestors carried placards with the messages: “We want Enoch Powell!”, “Enoch here, Enoch there, we want Enoch everywhere”, “Don’t knock Enoch” and “Back Britain, not Black Britain”.

The speech marked a watershed moment in the politics of race and immigration in Britain. It also ensured Powell would never again hold a senior political post and still defines his legacy.

Despite not holding a frontbench post, Powell maintained a high profile throughout the 1970 general election campaign. It has been suggested that Powell’s rhetoric helped contribute to the Conservative Party and Heath’s victory in 1970.

Powell left the Conservative party in February 1974 over his opposition to Heath’s europhile policies. He endorsed the Labour Party in the February 1974 general election over Harold Wilson’s commitment to a referendum.

In a speech in Birmingham on 23 February 1974, Powell claimed the main issue in the campaign was whether Britain was to “remain a democratic nation… or whether it will become one province in a new Europe super-state”.

Powell said it was people’s “national duty” to oppose those who had deprived parliament of “its sole right to make the laws and impose the taxes of the country.”

Powell then joined the Ulster Unionist Party (UUP) and stood in the October 1974 general election. He represented the seat of Down South from 1974 to 1992.

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In 1994, Alan Sked, the founder of the UK Independence Party (Ukip), asked Powell to stand for the party. Powell declined. He did, however, endorse three Ukip candidates in parliamentary elections between 1994 and 1997.

Powell had previously spoken in support of Sked when he stood as a candidate at the 1993 Newbury by-election on an anti-Maastricht platform.

Nigel Farage, an anti-Maastricht commodities trader, drove Powell to Newbury so he could participate in Sked’s by-election campaign. In March 1994, in his capacity as Ukip’s candidate in the Eastleigh by-election, Farage asked Powell for his endorsement.

According to the Telegraph, he wrote: “I have everything in place to fight a good, aggressive campaign but a voice from you could transform things and put the issue to the forefront. Please give us the help you can. As your performance at Newbury showed the electorate are beginning to wake from that long sleep. Come and give them another jog.”

Powell responded: “I have given very serious consideration to the request in your letter of 23rd March but have concluded that I do not feel I can contribute further to the campaign by speaking on your behalf at the by-election. Recent developments seem to show that opinion in the United Kingdom is consolidating against membership of the European Union.”

Powell died on 8 February 1998, aged 85.