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.”

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.”