On 14 July 1966, Plaid Cymru achieved its first-ever parliamentary breakthrough when its president, Gwynfor Evans, won the Carmarthen by-election, overturning a substantial Labour majority.

The by-election was called following the death of the long-serving Labour MP, Lady Megan Lloyd George, daughter of the former Liberal prime minister. Carmarthen had been considered a relatively safe Labour seat, held by Lady Megan since 1957, and Harold Wilson’s Labour government had secured a landslide national victory just four months earlier. Labour fielded Gwilym Prys Davies, a Welsh-speaking solicitor and former member of Plaid Cymru.

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Gwynfor Evans, president of Plaid Cymru since 1945, focused his campaign on the perceived neglect of Wales by successive governments. The slogans “For a better Wales” and “For work in Wales” were respectively targeted at rural and industrial portions of the constituency.

Another slogan urged voters to “vote for a free Wales”.

Evans previously stood in Carmarthen at the 1964 and 1966 general elections, coming third on both occasions. He finished 13,805 votes behind Megan Lloyd George in the 1966 general election.

As such, the July 1966 by-election produced a political earthquake. Evans secured 16,179 votes (39.0%), defeating Labour’s Gwilym Davies (13,743 votes, 33.1%) with a majority of 2,436. The swing from Labour to Plaid Cymru was a significant 12.0 points.

The Liberal candidate polled 20.8%, while the Conservative trailed with just 7.2%.

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Evans’s victory was a watershed moment for Plaid Cymru. It marked the arrival of Welsh nationalism as a significant electoral force, and provided a major boost for the Welsh language movement. The Welsh Language Act 1967 followed months later.

Upon arriving in Westminster on 21 July, Evans requested to take the oath of allegiance in the Welsh language “in view of the great national awakening” exhibited in Carmarthen.

The commons speaker, Horace King, responded that MPs should “address the House in English” referring to page 441 of Erskine May, the authoritative account of parliamentary procedure.

King stated: “The purpose of the rule is not to elevate the English tongue above others, but to ensure that the valuable contributions to debate which all honourable members make can be understood by the others.”

Evans protested.

He told the House: “May I say that this parliament is the only parliament that Wales has, that Wales is a nation with a national language that has been spoken there for nearly 2,000 years and that the people of Wales will regard it as an affront if that language cannot be spoken now in this House at least to take the oath?”

Labour MP Emrys Hughes cited the case of Charles Bradlaugh, who was repeatedly barred from parliament in the 19th century on account of his atheism, as evidence that the speaker should not always follow precedent in such matters.

The leader of the House of Commons, Herbert Bowden, intervened to suggest a select committee should consider the matter. “Perhaps this is an issue which we can consider sympathetically”, he stated.

Evans, denied the chance to take the oath in Welsh, delivered his maiden speech on 26 July.

Addressing the House, the Plaid Cymru MP argued that “governments at Westminster equate Britain with England that the impact of their policies on Wales is not considered.”

Evans stated that the Welsh nation should instead “be living with the dignity and fullness which the Scandinavian nations enjoy”.

He declared: “We had evidence in this chamber… that the English order has no place in it for a living Welsh nation. The rules of this English parliament prevent even the taking of the oath in the Welsh language.”

He suggested that the policy of successive governments to “assimilate the Welsh people” had reduced his nation “to ruin”.

Evans concluded: “Let Wales have the institutions of nationhood. Let Wales live like a nation and act like a nation. Today, she can do nothing for herself. She cannot even build a road for herself. She has no power of action, no power of decision or choice, and no freedom.

“Let the people of Wales be free to act for themselves and live their own lives rooted in the traditions of millennia, and rooted in their Christian values.”

Following the February 1974 election, members were permitted to take the oath in Welsh. Several MPs did so – but Evans was not among them. He lost his seat at the 1970 election and failed to regain it at the first time of asking.

Evans returned to parliament as the MP for Carmarthen at the October 1974 general election, when he took the oath in Welsh, serving until 1979. He failed to secure his return at the 1979 and 1983 general elections.

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Plaid Cymru’s Carmarthen breakthrough in 1966 – considered alongside the Scottish National Party’s success in Hamilton (1967) – led to the establishment of the Royal Commission on the Constitution (the Kilbrandon Commission) by the Labour government in 1969. The commission’s work constituted the first major study of the constitutional challenges posed by multi-national governance and rising national consciousness.

Its terms of reference required it to examine “the present functions of the legislature and government in relation to the several countries, nations and regions of the United Kingdom” and to consider “whether any changes are desirable in those functions or in existing constitutional and economic relationships.”

The report, published in October 1973 after a four-year inquiry, recommended legislative devolution for Scotland and Wales in the form of 100-member assemblies, elected for four-year fixed terms under the single transferable vote (STV) system. The number of Scottish and Welsh MPs at Westminster would be reduced from 71 to 51 and 36 to 31 respectively.

In September 1974, Wilson’s government published a white paper entitled Democracy and Devolution: Proposals for Scotland and Wales, putting forward proposals based on the Kilbrandon commission.

James Callaghan, who succeeded Wilson as prime minister in 1976, introduced the Scotland Act 1978 and the Wales Act 1978 – both of which provided for the establishment of devolved assemblies, subject to post-legislative referenda.

The Welsh devolution referendum held on 1 March 1979 saw voters in Wales decisively rejected the proposal to establish a devolved Welsh assembly – with the “No” campaign winning by a margin of four to one.

The more controversial outcome in Scotland triggered a series of events that led directly to the fall of the Labour government and the election of Margaret Thatcher.

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In 1980, Evans pledged to “fast to the death” if Thatcher’s new Conservative government did not reverse its opposition to a Welsh language television channel.

On this occasion, the lady was for turning.

Evans stepped down as Plaid Cymru president in 1981.

He died on 21 April 2005, aged 92.

In July 2016, a plaque to commemorate his famous by-election victory was unveiled in Carmarthen.