But Carl Bildt tells Aftonbladet that the information he passed on was already public knowledge at the time.
In the fall of 1976, three center-right parties were negotiating behind closed doors to form a government. Eleven days into the negotiations, on 1 October 1976, Thorbjörn Fälldin, the Center Party leader, told Aftonbladet that he was not in the position to provide any information about the negotiations.
But Aftonbladet reports that on the very same day the American ambassador to Sweden sent a secret report to the US Secretary of State.
Ambassador David Smith told Secretary Henry Kissinger about a meeting between a 27-year-old Carl Bildt and an American diplomat in Stockholm, Ronald E Woods. At the time Bildt was an assistant to the Moderate Party's, leader Gösta Bohman,
Aftonbladet reports that the cable, published by Wikileaks, says the information about the government negotiations came from a source "near the negotiations".
Bildt reportedly gave the US information about how the three party coalition would be created, plus the parties' stances on nuclear energy, and how they would handle a referendum on the issue.
Aftonbladet reports that the information in the report reached the US and the CIA before it became public knowledge in Sweden.
Ronald E Woods, the American diplomat who met Bildt, remembers the meeting. "He was well-informed. I reported in everything he said," Woods tells Aftonbladet.
Both the Center Party leader Thorbjörn Falldin and Per Ahlmark, the leader of the Liberal Party, say Carl Bildt was involved in the negotiations in 1976 to form a government. They tell Aftonbladet that the negotiations were very secretive.
"If I had known that Bildt had talked, then I would have been very irritated," Ahlmark tells Aftonbladet.
Bildt denies any wrongdoing. In a long blog post published today, he writes that the information the American diplomats sent to Washington was false in several ways. For example, Bildt says he did not give the Americans any information about the government's stance on nuclear energy.
"Totally normal diplomatic reporting. Completely trivial information. And nothing that happened that can be called 'secret'", Bildt writes on his blog.
And Wilhem Agrell, an intelligence and security expert, tells Aftonbladet that Bildt's contact with the US is not surprising. "This information doesn't affect the country's security so it doesn't come close to being classified," he says. "I don't think it's strange or unusual in any way."