US Supreme Court Justice Anthony Kennedy has announced he is retiring, giving President Donald Trump the chance to cement conservative control of the country’s top court.

The 81-year-old Justice Kennedy said on Thursday morning he is stepping down after more than 30 years on the court.

A Republican appointee, he has held the key vote on such high-profile issues as abortion, affirmative action, gay rights, guns, campaign finance and voting rights.

Justice Kennedy’s departure at the end of July will give Mr Trump an opportunity to appoint a more conservative judge to the nine-seat bench, potentially opening the way for a dramatic swing to the right for the court.

Without him, the court will be split between four liberal justices who were appointed by Democratic presidents and four conservatives who were named by Republicans.

Mr Trump said he has held Justice Kennedy in high esteem, and that a search for a replacement would begin immediately.

“He is a man who’s displayed great vision, he’s displayed tremendous vision,” he said.

Mr Trump and Justice Kennedy met shortly before the announcement was made.

“[We] had a very deep discussion. I got his ideas on things,” the President said from the Oval Office.

“I asked him if he had certain people he had great respect for that could potentially take his seat.”

Mr Trump’s nominee, expected to be named in weeks, is likely to give the conservatives a solid majority and will face a Senate process in which Republicans hold the slimmest majority. But Democrats cannot delay confirmation.

The President’s first High Court nominee, Justice Neil Gorsuch, was confirmed in April 2017.

Mr Trump already has a list of 25 candidates – 24 judges and Utah senator Mike Lee – from which he will choose a nominee.

Prominent on that list are judges Thomas Hardiman of Pennsylvania and William Pryor of Alabama, seriously considered for the seat eventually filled by Justice Gorsuch, and Judge Brett Kavanaugh, who serves on the federal appeals court in Washington, DC.

-with AAP