US Special Counsel Robert Mueller’s investigators met with a former British spy whose dossier on alleged Russian efforts to aid Donald Trump’s presidential campaign has spawned into months of widespread investigations, a media report said.

Informed sources told CNN on Thursday that information from Christopher Steele, a former MI-6 officer, could help investigators determine whether contacts between people associated with the Trump campaign and suspected Russian operatives broke any laws.

The Federal Bureau of Investigation (FBI) and the US intelligence community last year took the Steele dossier more seriously than the agencies have publicly acknowledged, the sources said.

James Clapper, then the director of national intelligence, said in a January 2017 statement that the intelligence community had “not made any judgment that the information in this document is reliable”.

Ever since the dossier came to light in January, Trump and his allies have repeatedly insisted that it is a complete work of fiction.

He told The New York Times earlier this year that the dossier “was totally made-up stuff”.

In a series of tweets, Trump said the memos were written by a “failed spy” who had relied on “totally made-up facts by sleazebag political operatives”.

The dossier is a collection of memos that were initially intended as political opposition research, the sources told CNN.

Steele was hired mid-2016 by a Washington firm that was already collecting opposition research about Trump.

At a news conference this week, Senator Richard Burr, chairman of the Senate intelligence committee, expressed frustration that Steele has rebuffed efforts to meet with him.

“The committee cannot really decide the credibility of the dossier without understanding things like, who paid for it? Who are your sources and sub-sources?.

“We’re investigating a very expansive Russian network of interference in US elections. And though we have been incredibly enlightened at our ability to rebuild backwards, the Steele dossier, up to a certain date, getting past that point has been somewhat impossible.”