On 17 June 1974, a bomb planted by the Provisional IRA exploded in Westminster Hall, the historic heart of the Houses of Parliament, injuring 11 people and causing significant structural damage.

The attack was part of an escalating IRA bombing campaign on the British mainland intended to pressure the UK government into withdrawing from Northern Ireland. It took place just three weeks after the collapse of the Sunningdale Agreement, which provided for a power-sharing executive in Northern Ireland.

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The device, estimated to be 20lbs (9.1 kg) of explosives, had been hidden in a women’s toilet adjoining the 900-year-old hall. It detonated at around 8:28 am. A warning was telephoned to the Press Association offices just six minutes before the explosion, which was not enough time to complete a full evacuation. Police said the message, delivered in an Irish accent, included a recognised IRA codeword.

The casualties were mostly parliamentary staff who were preparing for the day’s sitting. The blast caused extensive damage. For several hours smoke shrouded Big Ben as firemen fought the blaze fed by an ignited gas main.

The Grand Committee room, which is today the site of Westminster Hall debates, was also damaged. There was no damage to the House of Commons or the House of Lords.

The bombing of the seat of British democracy prompted an immediate review of security at the Palace of Westminster, announced by Edward Short, the leader of the House of Commons.

Short told MPs: “Just before 8.30 this morning a bomb, which is estimated as weighing 15–20 lb., exploded near the north end of Westminster Hall.

“Six minutes’ warning was given by a telephone call to the Press Association, and officers from New Scotland Yard were already in the precincts when the explosion took place. But the warning related to the House of Commons and was quite unspecific…

“Unfortunately five members of the staff of the House and one fireman were injured. One of the members of the staff is being treated in hospital for a broken leg.”

The commons leader announced a “review the whole of our security procedures, because of the incident, and that process will begin tomorrow.”

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Jeremy Thorpe, the leader of the Liberal Party, responded: “This House will be not bombed into a decision, but will negotiate peacefully with men of peace.”

Robert Mellish, the Labour chief whip, said that “if they had blown up the House of Commons, it would not stop us from our work.”

Queen Elizabeth II issued a statement saying she was “greatly shocked by the explosion” and “distressed about the injuries and damage this has caused.”

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The attack was followed just over a month later by a fatal bombing at the Tower of London, and attacks in Birmingham and Manchester.

In the year of 1974, the IRA killed 28 people (23 civilians and 5 British soldiers) in bombing operations in England.

Five years later, on 30 March 1979, a car bomb claimed the life of Airey Neave, the shadow Northern Ireland secretary, while he was driving out of the commons car park. The Irish National Liberation Army (INLA) were responsible for the assassination.