The United States President Donald Trump has disclosed that the search for a replacement for retiring Supreme Court Justice Anthony Kennedy will "begin immediately".

"We have to pick one that's going to be there for 40 years, 45 years," Mr Trump said at a rally in North Dakota.

The retirement of Mr Kennedy, a conservative who sided with liberals on many votes, gives Mr Trump the chance to shift the top court's balance more to the right for decades to come.

The judge, 81, will retire on 31 July.

He made the announcement on Wednesday, saying he wanted to spend more time with his family after 30 years on the top court. Mr Trump later praised Mr Kennedy - who held the pivotal vote on many key cases - as "a great justice of the Supreme Court".

"Hopefully we are going to pick somebody who will be as outstanding," he said at the White House. The judge's retirement gives Mr Trump his second Supreme Court pick since he became president, and he has said he will choose from a list of 25 conservative candidates.

The Supreme Court plays a key role in American life and is often the final word on highly contentious laws, disputes between states and the federal government, and final appeals to stay executions.

This week the court upheld Mr Trump’s travel ban which covers people from several Muslim-majority countries, in a 5-4 conservative majority ruling. Earlier this month it ruled in favour of a baker in Colorado who refused to make a wedding cake for a gay couple.

Speaking at the rally in Fargo, North Dakota on Wednesday evening, President Trump told supporters that Mr Kennedy had chosen to retire under his presidency "because he felt confident in me to make the right choice and carry on his great legacy".

The president has promised to draw names from the same list from which he picked Neil Gorsuch in February last year.

Rather than serving fixed terms, the justices serve for life unless they decide to retire. This makes their appointments particularly significant.

Mr Kennedy, who is the second-oldest justice on the nine-member US Supreme Court, earned a reputation as a swing vote conservative who supported liberal arguments on key decisions, including the 5-4 rulings that decided same-sex marriage and upheld abortion rights.

As a result, news of his retirement has raised fears among pro-choice groups that access to legal abortions in several states could be under threat