Donald Trump's decision to declassify evidence given by ex-British spy Christopher Steele over the former U.S. president's alleged links with Russia led to the disappearance of two sources, Steele said in court documents made public.

Trump, the frontrunner for the 2024 Republican presidential nomination in the U.S., is suing Orbis Business Intelligence, a British private investigations firm co-founded by Steele, over the "Steele dossier" in order, he said in his own witness statement, to prove its claims were false.

The dossier, published by the BuzzFeed website in 2017, alleged ties between Trump's campaign and Russia, and said Trump engaged in sexual behaviour that gave Russian authorities material with which to blackmail him.