Donald Trump took “British naval secrets” to Mar-a-Lago after he left the White House, the former UK spy Christopher Steele says in a new book. “I was reliably informed by impeccable sources that among the classified documents which Trump, apparently unauthorizedly, took with him to Mar-a-Lago at the end of his presidency were British naval secrets, some of the most sensitive ones in our governmental system,” Steele writes. “It remains unclear to me, at least, why Trump would have wanted to retain such documents and what eventually happened to them.” Steele does not say what the secrets concerned. In a statement sent to the Guardian after this story was published on Tuesday, a spokesperson for the British Ministry of Defence said of Steele’s comments about naval secrets taken to Mar-a-Lago: “These claims are untrue.” A former MI6 agent, Steele became famous as the author of a dossier of often unverified information on Trump and his links to Russia which caused a media sensation shortly after the 2016 election, in which Trump beat Hillary Clinton in a historic upset. The dossier has been the subject of controversy ever since. But a US investigation under the special counsel Robert Mueller did establish that Russia interfered in the election to boost Trump, detail extensive links between the former president and Moscow, and produce numerous indictments and convictions. Steele’s book, Unredacted: Russia, Trump and the Fight for Democracy, was published on Tuesday – a little over a month from election day as Trump runs to return to the Oval Office, against the Democratic candidate, Kamala Harris. The race unfolds amid warnings of further Russian efforts to help him, including from Mueller himself. Trump’s retention of classified documents after leaving power in 2021 was the subject of an FBI raid on Mar-a-Lago and 40 criminal charges brought by the special counsel Jack Smith. That case was thrown out in June by Aileen Cannon, a Florida judge appointed under Trump. Smith has appealed. Trump was convicted on 34 criminal charges in New York, over hush-money payments to an adult film star. His attempts to overturn the 2020 election are the subject of four federal criminal charges and eight in Georgia. Trump was previously reported to have discussed US nuclear submarines at Mar-a-Lago with an Australian billionaire who then shared the information. Reporting of that incident did not mention “British naval secrets”. At the time of Trump’s 2016 win, Steele was running Orbis Business Intelligence. BuzzFeed published the Trump dossier. Sensational unverified allegations, including placing Trump with sex workers in a Moscow hotel, ensured blanket coverage. Trump angrily rejected the dossier and sued Steele. The case was thrown out this year, and Trump was ordered to pay costs, an order Steele says has not been met. Steele now writes that the day after the 2016 election, he felt “disturbed” and “afraid – for democracy, for the United States, for the world at large”, as he felt his warnings about Trump and Russia had fallen on deaf ears. “We were no longer even on good terms with the FBI. We had tried to warn them, and the public, about Trump and Russia and we had failed. How long would it be before Trump himself found out about our reporting – and about us? How would he respond? “‘Chaos is about to break out,’ I thought.” He and his partner, he says, felt they had “a fresh duty: we needed to tell the British government [about our work] because Trump’s election could pose a direct threat to UK national interests … if Trump were compromised, the British government needed to know”. Steele says he visited Charles Farr, chair of the British joint intelligence committee, who circulated a summary of Steele’s findings “among the key people in government including senior British officials and ministers”. But, Steele writes, repeating testimony to MPs, the Conservative government then led by Theresa May chose to “sit on the information and avoid alienating the incoming American president”. Steele says he had briefed May before but “she was clearly worried about alienating Britain’s most powerful ally. It was cowardly behavior, in my opinion, and wrong. There is being close to the United States, and then there is being close to the United States to the extent that one’s own national security is jeopardized.” Steele discusses the reported occasion in May 2017 when Trump shared “top secret intelligence”, allegedly about an Israeli asset working inside the terrorist group Isis, with Sergei Lavrov, the Russian foreign minister. Sources, Steele says, believe Russia told Iran, which tipped off Isis, leading to the death of the asset. Steele then cites his “impeccable source” saying Trump took documents concerning British naval secrets to Mar-a-Lago. He also says the British government knew of links between Trump and Moscow before his intervention, as the Australian government reported activities involving a Trump adviser, George Papadopoulos, in 2016. A long-delayed British report on Russian influence was published in 2020, by which time May had been succeeded as prime minister by Boris Johnson. The Labour party, then in opposition, said the report authors, a cross-party group of MPs and peers, had shown “the scale of the shortcomings of the government’s response to maintaining our national security in the face of what is clearly a growing and significant threat from Russia”. Steele calls May’s decision to stay close to Trump “at best, denial and at worst, irresponsible. The British government had decided to pretend that Trump was a normal president-elect, that he had not been helped in his victorious election campaign by Russia, and that he and his team could be trusted with the most sensitive, life-and-death information that governments hold. As we later saw, they could not.” Speaking to the Washington Post, which interviewed Steele this week, Trump’s spokesperson, Steven Cheung, said: “Any new information by this foreign agent who peddled the debunked Steele dossier should be wholly dismissed, and any media outlet that entertains anything he has to say is just the continuation of election interference intended to meddle in the campaign.” Steele told the Post he had “weighed the risks – including the risk of harm to Trump” arising from publishing new unverified claims, such as that about British naval secrets allegedly taken to Mar-a-Lago. But Steele added that he had decided it was in the public interest to do so, even “if the allegations I reference in the work are untrue”.