Senior officials at the Foreign Office repeatedly warned No 10 that Rishi Sunak should not leave June’s D-day commemoration in Normandy early, according to new revelations in a book about the Tories’ 14 years in power. The department passed on two messages to Downing Street in the weeks leading up to the event, which were then ignored in what has gone down as the worst election campaign blunder of the last 14 years. The claim is contained in the paperback version of Blue Murder, by the Daily Telegraph’s political editor, Ben Riley-Smith. The Guardian has also spoken to multiple sources about the events of that day, giving the fullest picture yet of the mistake that came to define Sunak’s campaign and taint his entire premiership. According to the book, the Foreign Office provided written advice on two occasions before the event telling Downing Street that the prime minister should attend. The first came a few weeks before the event and the second just a few days before, once it became clear that Keir Starmer would be attending, as would the Ukrainian president, Volodymyr Zelenskiy. One source said: “There was very clear advice from the department that it would be an important event to go to and there would be significant risks should he not attend.” Another said: “The Foreign Office reiterated clear advice that the PM should confirm attendance at the international ceremony.” Having endured a difficult start to the campaign, Sunak had regained some momentum with a combative first debate performance against Starmer. But as the 80th anniversary D-day commemoration approached on 6 June, his senior team had no sense of the PR disaster that was about to engulf them. Sunak’s decision to return home halfway through the day’s events to resume his election campaign was widely criticised, and undermined his own claim that only the Tories could be trusted to protect Britain’s security. The photograph of three world leaders – Emmanuel Macron of France, Joe Biden of the US and Olaf Scholz of Germany – alongside the British foreign secretary at the time, David Cameron, was to become one of the abiding images of the campaign – and Sunak’s premiership. Sources have told the Guardian that Sunak always planned to attend the D-day event but believed the only essential part of the day was the British-led ceremony in the morning. He was bolstered in this belief by his campaign advisers, some of whom advised him not to attend the afternoon event at Omaha beach as King Charles, who was undergoing treatment for cancer, would not be there. “We were worried that if the PM went when the king was not going it would look like we were trying to upstage him,” said one Conservative official. Accounts differ as to when it became clear that other world leaders would be at the afternoon commemoration. One Sunak ally said it only became clear a few days beforehand that Biden and Scholz would join Macron on the beach. However, a French official told reporters in Paris a week beforehand, on 30 May, that it would be attended by both men, as well as heads of government from Denmark, Luxembourg, Greece, the Netherlands, Norway and Canada. The Élysée also believed that Sunak would attend, although the official added that his presence was “not totally certain”. Despite the British government having a week’s notice of the event’s high-profile attenders, Sunak’s officials continued to labour under the misunderstanding that it would be a low-key event. “It seemed like they were inviting us to a cocktail party,” said one. “It was not the kind of thing we thought it was worth us interrupting a general election campaign for.” That belief contradicted the message coming from Paris, where the government official told reporters on 30 May that the Omaha beach event would be “a ceremony which will be structured by readings, testimony and artistic performances, and will be sombre and very symbolic”. Multiple government sources have told the Guardian that they believed the problem was that the election campaign meant Sunak did not have regular contact with his civil service advisers, who might have made sure he had all the necessary information. “It is difficult in a campaign, as the prime minister does not have the kind of regular official-level briefings he is used to getting,” said one. Despite all this, Sunak’s absence may still have gone largely unnoticed had it not been for Macron’s decision to pose for an impromptu photograph alongside Scholz and Biden. Having spotted David Cameron, the sources said, the French president pulled him into the photograph too – even though the UK’s official attender was Grant Shapps, the defence secretary. Footage of the four men posing was played on British television screens and quickly went viral on social media. The impression of Sunak having snubbed three of the world’s most important people was reinforced when it emerged he had instead taken part in an election interview with ITV back in London. Sunak’s advisers realised how bad the situation looked by the following morning, according to Riley-Smith’s book, and persuaded him to tweet a written apology. “On reflection it was a mistake not to stay in France longer, and I apologise,” Sunak wrote. The fallout continued, the book reveals, with several cabinet ministers complaining that the apology had made things worse by giving the story extra fuel. Sunak was to spend much of the rest of the campaign apologising for his decision, which Tory aides say he also accepted responsibility for within his own team. In the end, the former prime minister was never able fully to shake off the controversy, which is now remembered alongside Gordon Brown in 2010 calling a voter a “bigoted woman” as one of modern history’s great campaign errors. One source told Riley-Smith: “Basically it was just a massive fuck-up.”