Hillary Clinton has seen her opponent maintain his lead
Hillary Clinton will withdraw from the race to become the Democratic candidate for the US presidency, and back her rival Barack Obama, her campaign says.
On Tuesday, Mr Obama gained enough delegates to win the nomination, after the final votes of the primary season.
Mrs Clinton has not publicly admitted losing, but at an event on Saturday she will do so "and express her support for Senator Obama", aides say.
Mr Obama has already announced a team to help select his running mate.
Reports that Mrs Clinton was ready to concede came after she made a conference call to senior Democrats in Congress.
At a Democratic Party event in Washington, Mrs Clinton will also "express her support... for party unity", her communications director Howard Wolfson said.
Earlier, it had been announced that the event would be held on Friday, but Mr Wolfson said it had been delayed a day "to accommodate more of Senator Clinton's supporters who want to attend".
The BBC's Jane O'Brien in Washington says that as Mr Obama was claiming victory, Hillary Clinton stunned even her own supporters with a speech that offered no indication that she was giving up.
There is speculation that the delay in conceding was an attempt to position herself as a possible vice-president, our correspondent adds.
Mr Obama's three-member panel to look for a presidential running mate comprises Caroline Kennedy, daughter of President John Kennedy, former deputy Attorney General Eric Holder and Jim Johnson.
Mr Johnson performed the same selection task for John Kerry in 2004.
"Senator Obama is pleased to have three talented and dedicated individuals managing this rigorous process," said Bill Burton, a spokesman for the Illinois senator.
"He will work closely with them in the coming weeks but ultimately this will be his decision and his alone."
Earlier, Mr Obama had paid tribute to Mrs Clinton and hinted that she would play a role in any future Obama administration.
Mrs Clinton has said she would be "open" to the idea of being Mr Obama's vice-presidential running-mate.
Referring to a brief conversation he had held with his defeated rival, the Illinois senator said: "I'm very confident of how we're going to be able to bring the party together."
The Republican party's candidate, John McCain has challenged Mr Obama to take part in debates in 10 town hall meetings before August's Democratic convention, and the Obama team is said to be considering the invitation.
The final primaries of the season were held on Tuesday - with Mr Obama winning Montana and Mrs Clinton winning South Dakota.
A candidate needs 2,118 delegates to secure the nomination and Mr Obama now has the support of 2,154 delegates. Mrs Clinton has 1,919.