The most senior US intelligence official told a Senate oversight panel that he “simply didn’t think” of the National Security Agency’s efforts to collect the phone records of millions of Americans when he testified in March that it did “not wittingly” snoop on their communications.

James Clapper, the director of national intelligence, made the comments in a letter to the Senate intelligence committee, released in full for the first time on Tuesday.

Portions of the letter, in which Clapper apologised for giving “clearly erroneous” testimony at a March hearing of the committee, were first reported by the Washington Post on Monday. Clapper had previously said that his answer to the committee was the “least untruthful” one he could publicly provide.

In the full letter, Clapper attempted to explain the false testimony by saying that his recollection failed him. “I simply didn’t think of Section 215 of the Patriot Act,” he wrote to committee chairwoman Dianne Feinstein (Democrat, California) on 21 June, referring to the legal provision cited to justify the mass collection of Americans’ phone data, first disclosed by the Guardian.

Clapper is under intense pressure from legislators displeased by his March testimony to the Senate intelligence committee’s Ron Wyden (Democrat, Oregon) that the NSA did “not wittingly” collect, as Wyden put it, “any type of data at all on millions or hundreds of millions of Americans.”

In his newly released letter, Clapper told Feinstein that his remarks were “clearly erroneous,” and he issued them because he was thinking instead of a different aspect of surveillance, the internet content collection of persons NSA believes to be foreigners outside of the United States.

“I apologize,” Clapper wrote. “While my staff acknowledged the error to Senator Wyden’s staff soon after the hearing, I can now openly correct it because the existence of the metadata program has been declassified.”

In statements for the past month, Wyden and his staff have said they told Clapper before the fateful hearing that he would face the question, and contacted his staff afterward to correct the record.

“The ODNI [Office of the Director of National Intelligence] acknowledged that the statement was inaccurate but refused to correct the public record when given the opportunity. Senator Wyden's staff informed the ODNI that this was a serious concern,” Wyden spokesman Tom Caiazza said on Monday.

Clapper’s letter does not acknowledge that he had earlier told Andrea Mitchell of NBC News that he provided Wyden with the “least most untruthful” answer he could publicly offer, likening the question “in retrospect” to a “stop beating your wife kind of question.”

A spokesman for Clapper declined to comment on the discrepancy.

Clapper has said in the past that public testimony on intelligence matters places spymasters in difficult positions. “An open hearing on intelligence matters is something of a contradiction in terms,” Clapper told the Senate intelligence panel on March 12, while saying he believed it was “important to keep the American public informed.”

Clapper is under fire from legislators critical of his truthfulness. On Friday, 26 senators – more than a quarter of the Senate – signed a letter to Clapper suggesting that the surveillance may go beyond phone records and online communications, extending under interpretations of the Patriot Act to “credit card purchases, pharmacy records, library records, firearms sales records” and more.

But Clapper has his supporters as well. In addition to the White House, which is standing beside him, a former NSA lawyer and inspector general, Joel Brenner, wrote on Tuesday that Wyden engaged in a “vicious tactic” that “sandbagged” Clapper.

Wyden “lacked the courage of his conviction,” Brenner wrote on the influential national-security blog Lawfare, and placed Clapper “in the impossible position of answering a question that he could not address truthfully and fully without breaking his oath not to divulge classified information.”

It is unclear when Clapper will publicly testify next. He sat out the House intelligence committee’s June 19 hearing on the NSA surveillance. Aides to Feinstein said that no hearing with Clapper is currently scheduled, although Feinstein is open to one.

The next opportunity for one might come as early as next week. Clapper told Mitchell on June 9 that Feinstein had asked him to look at “ways where we can refine these [surveillance] processes and limit the exposure to Americans’ private communications,” adding that “we owe her an answer in about a month.”

A spokesman for Clapper had no comment on the director’s progress in examining restrictions to the surveillance efforts.