Former independent counsel Ken Starr on Monday sharply criticized the leak of Special Counsel Robert Mueller's letter to Attorney General William Barr following Barr's summary of the Russia report, calling the release of the letter an "unforgivable sin."

"His letter, that was then leaked on the very eve of Bill Barr's testimony, was essentially, I believe, an unfair, whiny complaint," Starr argued on "The Story with Martha MacCallum."

Details of the March 24 letter went public shortly before Barr testified before the Senate Judiciary Committee in a tense hearing last week. Multiple news outlets reported that Mueller raised concerns about how Barr's conclusions from the investigation were being portrayed, before the Justice Department had released a redacted version of Mueller's report.

Starr also said Barr had been "obliged" to summarize Mueller's findings.

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Democrats have urged Mueller to testify after his March letter leaked.

Barr skipped a House Judiciary Committee hearing last Thursday over the terms of the session, though he testified for hours a day earlier on the Senate side. He also has refused requests for the unredacted Mueller report, though he has offered access to a less-redacted version to certain lawmakers.

House Judiciary Committee Chairman Jerrold Nadler, D-N.Y., in a letter to Barr last Friday, threatened to launch contempt proceedings and "seek further legal recourse" should the attorney general and Justice Department continue what he called a "baseless refusal to comply with a validly issued subpoena" for the full Mueller report.

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Starr said Barr was trying to do the right thing, only for Mueller to set him up for criticism.

"Here comes Bob Mueller with this letter which is then leaked. That is, to me, the unforgivable sin. He, Bob Mueller, badly injured this attorney general and the attorney general didn't deserve that but, of course, that created its own huge firestorm including suggestions that the attorney general was totally mischaracterizing the report and so forth," Starr said.

Fox News' Brooke Singman contributed to this report.

Fox News' Martha MacCallum contributed to this report.