WASHINGTON (CNN) -- White House counsel Harriet Miers, once President Bush's pick for a seat on the U.S. Supreme Court, has resigned, the White House announced Thursday.

Bush has "reluctantly accepted" her resignation, which takes effect Jan. 31, the White House said.

Her resignation letter, dated Thursday, gave no explanation for her departure.

"It is hard to leave the tasks at hand," Miers wrote. "They are gravely important, and I shall miss greatly the arena in which the battles of our time are being fought. Please know, though, that I will remain a committed advocate and am at your service."

Miers was Bush's personal lawyer in Texas before joining the White House staff in 2001 and became White House counsel in 2005. In October of that year, Bush picked her for a Supreme Court vacancy, but Miers withdrew from the confirmation process after a storm of criticism.

Mier's resignation letter