Almost exactly half a century ago, a by-election was about to take place whose surprise outcome would shape the politics of Britain for the rest of the decade.

From the moment Labour won the general election of October 1964 by a majority of just four, there was speculation the party would be forced to hold a second election fairly soon. It was anticipated this would happen as the result of Labour losing sequentially its majority in the Commons and a vote of confidence.

This scenario edged even closer to reality on 7 November 1965 with the death of Henry Solomons, the Labour MP for Hull North. His seat was the most marginal to fall vacant since the 1964 election, when Labour had won Hull North with a majority of just 1,181. The Tories needed a swing of just over 1% to retake the constituency.

The Hull North by-election was set for 27 January 1966, and right up to polling day local press reported that Labour was likely to lose the seat. But in the event, and to almost everyone’s surprise, Labour not only held on but increased its majority to 5,351 on a swing of 4.4%: the largest swing to a governing party in a marginal seat in any by-election since 1924:

One factor that may or may not have influenced the outcome was the announcement during the campaign of the construction of the Humber Bridge, allegedly prompted by Harold Wilson calling in a favour from his transport secretary Barbara Castle.

The Hull North result did not mark an end to speculation about the timing of the general election, however. If anything it intensified, as the press suspected – rightly – that Labour was now more inclined to go the country sooner rather than later. Just one week later events brought the subject to head when another Labour MP died: Harold Hayman, who, like Solomons, held a marginal seat: Falmouth & Camborne.

The only way for Labour to avoid holding another by-election was to call a general election. Initially Harold Wilson, the prime minister, played things rather cryptically. He refused to talk publicly about a general election, and even went off an official visit to Moscow on 21 February. There was also the matter of the Budget, which was due at some point in March. Should this be rushed through before an election, or put on hold until afterwards?

Wilson revelled in this kind of situation. He liked little more than holding lots of cards in his hand and keeping his friends (and foes) guessing. The Sunday Times on 30 January quoted a colleague of Wilson as saying: “It’s ideal for Harold, because it justifies him in not making his mind up on the election.”

Tony Benn captured a flavour of the mood of the times in his diary:

[Hull North] has created a completely new political situation. [Edward] Heath is in serious trouble with his own party and the criticism there is bound to grow. Harold is sitting pretty with a majority restored to its full strength and a retained option on the date of the election. Peter Shore is in favour of dissolving immediately, Dick Crossman of going in March or May, but I’m not sure Harold will do it. Much of our support in the country now comes from the fact we are carrying on and we haven’t yet written such a firm record as to make it absolutely certain we would be returned. Moreover the Tories and Liberals will now be reluctant to defeat us in the Commons, even if that were possible.

In the event the Budget was postponed and an election called for 31 March, which Labour went on to with a landslide majority of 96, remaining in office for the rest of the decade.

One final note, as recorded by David Butler and Anthony King in their study of the 1966 election:

Mr Wilson original intended to remain aloof from the campaign until the last week, but pressure from the party and consciousness of the risk he was taking finally led him to accept speaking engagements for every evening from March 11th onwards…

When viewed from today, the thought of a prime minister choosing not to play a leading role in an election campaign is just remarkable. Then again, some may consider it wishful thinking.