A judge in Washington DC has dismissed a lawsuit against an ex-British spy who compiled a dossier alleging links between Donald Trump and the Kremlin.

Christopher Steele was sued by three Russian oligarchs who claimed he defamed them by writing that they tried to influence the 2016 US election.

Mr Steele's lawyers argued that the legal action was frivolous and an attempt to silence him.

On Monday a judge agreed, saying Mr Steele has a right to his opinion.

Lawyers for Russian billionaires Mikhail Fridman, Petr Aven and German Khan had argued that Mr Steele was not entitled to free speech protections under the first amendment to the US constitution, because he is not a citizen of the United States.

But Judge Anthony Epstein disagreed, writing in his judgment that "advocacy on issues of public interest has the capacity to inform public debate, and thereby furthers the purposes of the First Amendment, regardless of the citizenship or residency of the speakers".

The judge also cited Washington DC's anti-SLAPP law (or strategic lawsuits against public participation), which prevents parties from filing lawsuits to tamp down on opposing political views.

Mr Steele's American lawyer, Christy Hull Eikhoff, said the court had shown support for the first amendment and the "profound national principle" that debate should be "uninhibited, robust and wide open".

Lawyers for the Russian billionaires told the BBC in a statement they would probably appeal the decision.

The three billionaires have previously brought a defamation case against Fusion GPS, the Washington DC-based research company that hired Mr Steele.

The dossier - containing salacious allegations against President Donald Trump - was first published BuzzFeed in January 2017.

President Trump has repeatedly dismissed the Steele dossier on Twitter as "fake" and "fraudulent".

He has also called Mr Steele a "lowlife" and claimed that the document was financed by his 2016 presidential rival Hillary Clinton.