LOS ANGELES (Reuters) - Republican presidential candidate Mitt Romney will pick up the formal endorsement of former President George H.W. Bush during a visit to Houston on Thursday, a Romney campaign official said.

Bush, 87, who was U.S. president from 1989 to 1993, has spoken warmly of Romney during the campaign but had withheld a formal endorsement of the former Massachusetts governor.

"Mitt and Ann (Romney) are close friends with George and Barbara, and governor Romney is honored to have the president's support," a Romney campaign official said.

Bush's son, former president George W. Bush, has not endorsed a candidate in the Republican primary race, preferring to stay out of the contest to pick a challenger to Democratic President Barack Obama in the November 6 election.

The elder Bush's endorsement adds to a long list of establishment Republicans who are supporting Romney, whose struggles to persuade party conservatives to back his campaign have prolonged a nominating race in which he has long been seen as the favorite.

(Reporting by Steve Holland; Editing by Vicki Allen)