The announcement came after Libby was indicted on Friday on five criminal counts of obstruction of justice, perjury and making false statements after a two-year investigation into the leak of a covert CIA operative's identity.
President George Bush's top political adviser, Karl Rove, was not indicted along with Libby but special counsel Patrick Fitzgerald has made clear to Rove that he remains under investigation and in legal jeopardy, legal sources said.
Libby, who as Cheney's chief of staff played a major behind-the-scenes role in building the case for the Iraq war, was accused of lying in 2003 about how and when he learned and disclosed to reporters classified information about the covert operative, Valerie Plame.
If convicted, Libby, 55, faces a maximum sentence of 30 years in prison and a $1.25 million fine, prosecutors said.
Fitzgerald planned to hold a news conference at 2pm (1800 GMT).
The charges handed up by the grand jury during a three-minute proceeding on Friday were the first in the two-year investigation, sparked by the public disclosure of Plame's identity in a 14 July 2003 newspaper column by Robert Novak.
The leak case has put a spotlight on the sometimes aggressive tactics the White House uses to counter critics of the Iraq war. It has also focused attention on the administration's shifting justifications for the 2003 invasion, from the threat of Iraqi weapons of mass destruction - which were never found - to a need to spread democracy.
Plame's identity was leaked to the media after her diplomat husband, Joseph Wilson, accused the Bush administration of twisting prewar intelligence to support military action against Iraq. Wilson said it was done deliberately to erode his credibility.