WATCH LIVE: Trump's Supreme Court appointee Neil Gorsuch to be sworn in on Monday

Neil Gorsuch, U.S. President Donald Trump's Supreme Court appointee, is due to be sworn in on Monday morning with a formal appearance at the White House, marking the biggest triumph so far for the new administration.

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The lifetime appointment reinstates the nine-seat court's 5-4 conservative majority, fulfilling an important Trump campaign promise.

"He will be a great Justice," Trump said in a Twitter post on Saturday. "Very proud of him!"

Gorsuch, 49, was the youngest Supreme Court nominee since Republican President George H.W. Bush in 1991 picked Clarence Thomas, who was 43 at the time. Gorsuch could be expected to serve for decades, while Trump could make further appointments to the high court to make it even more solidly conservative because three of the eight justices are 78 or older.

Gorsuch, whom the Senate confirmed on Friday, will take his judicial oath at 11 a.m. EDT in a Rose Garden ceremony. It will be administered by Supreme Court Justice Anthony Kennedy, for whom Gorsuch clerked as a young lawyer.

Gorsuch will become the first justice to serve alongside a former boss.

At 9 a.m. EDT, Gorsuch is due to take his separate constitutional oath, administered by Chief Justice John Roberts, in a private ceremony at the Supreme Court .

The Senate, which last year refused to consider Democratic former president Barack Obama's nominee to the court, on Friday voted 54-45 to approve Colorado-based federal appeals court judge Gorsuch. The vote brought to an end to an almost 14-month battle over a vacancy created by the death of conservative justice Antonin Scalia in February 2016.

Once sworn in, Gorsuch can prepare for the next round of oral arguments, starting on April 17, at the court, whose current term ends in June.

He will also participate in the justices' private conference on Thursday to consider taking new cases. Appeals are pending on expanding gun rights to include carrying concealed firearms in public, state voting restrictions that critics say are aimed at reducing minority turnout, and allowing business owners to object on religious grounds to providing gay couples certain services.

Gorsuch could also play a vital role in some cases on which his new colleagues may have been split 4-4 and therefore did not yet decide. Those cases may have to be reargued in the court's next term, which starts in October.

(Reporting by Lawrence Hurley; Editing by Lisa Von Ahn)