Image copyright PA Image caption Christopher Steele says his firm had a "commercial relationship" with the FBI

A former British spy has hit back after his research was criticised by a US watchdog's inquiry into FBI wiretapping of a Trump election campaign adviser.

A US Department of Justice inspector general on Monday faulted FBI reliance on Christopher Steele's dossier.

In a six-prong rebuttal, Mr Steele defended his work, which surfaced before the 2016 White House election.

The so-called Steele dossier made largely unsubstantiated claims linking President Donald Trump to the Kremlin.

The former MI6 officer was hired to conduct the research through a law firm on behalf of Mr Trump's political opponents, including Hillary Clinton's campaign.

What did the watchdog say?

Inspector General Michael Horowitz found the FBI did not act with political bias when it applied for a surveillance warrant on former Trump campaign adviser Carter Page, but that it made "serious performance failures" during the process.

The watchdog said the FBI "overstated the significance" of Mr Steele's past work when seeking the wiretap warrant.

The report noted the CIA itself viewed the Steele dossier as little more than "an internet rumour".

The inspector general notes that Mr Steele also heard salacious allegations from his Russian source network that he did not pass on to the FBI.

He cited an unsubstantiated claim that Mr Trump had once participated in sex parties in St Petersburg.

On Tuesday, Attorney General William Barr echoed the inspector general's criticism of FBI reliance on Mr Steele's work.

America's top law official called the dossier "complete rubbish" and a "sham". Mr Trump has called the dossier "a hoax".

How has Steele responded?

In a lengthy statement issued through his lawyers on Tuesday, Mr Steele accused the inspector general of making "serious errors".

It maintained that key claims in his dossier had been corroborated by Mr Page.

The statement points out that Mr Page admitted having met an associate of Igor Sechin, the president of Russian oil company Rosneft. Mr Sechin is considered a close ally of Russian President Vladimir Putin.

However, the Steele dossier asserts that Mr Page met Mr Sechin himself, an allegation that has not been confirmed by US authorities.

The statement says Mr Steele's London-based private intelligence agency, Orbis, had a "commercial relationship" with the FBI.

How did Ivanka Trump come up?

Mr Steele conducted an interview with the inspector general for the justice department report, dismissing Republican suggestions that he was biased against Mr Trump as "ridiculous".

The watchdog's report notes: "He [Mr Steele] stated if anything he was 'favourably disposed' toward the Trump family before he began his research because he had visited a Trump family member at Trump Tower and 'been friendly' with that person for some years."

Mr Steele had described the relationship as "personal", according to the inspector general.

This family member has been identified by US media as Mr Trump's eldest daughter, Ivanka Trump.

Mr Steele and Ms Trump reportedly met in 2007 and remained friends until 2015.

Image copyright Win McNamee/Getty Images Image caption Ivanka Trump was reportedly friends with former British Spy Christopher Steele

What else have we learned recently about the Steele dossier?

In last month's Trump impeachment hearings, former White House Russia expert Fiona Hill testified there is a chance Mr Steele had received false information from the Russians.

"It's very likely that the Russians planted disinformation in and among other information that may have been truthful, because that's exactly, again, the way that they operate," she said in closed-door testimony.

Operatives from Fusion GPS, the Washington DC-based research company that hired Mr Steele to compile the dossier, have written a new book about the affair.

They write that "many of the allegations in the dossier have been borne out".

But they concede that "other details remain stubbornly unconfirmed, while a handful now appear to be doubtful, though not yet disproven".

What is verified or unsubstantiated in the Steele report?

Department of Justice Special Counsel Robert Mueller backed the dossier's overarching assertion that Russia sought to back Mr Trump in 2016's election, but he discounted other claims.

Mr Mueller did not establish that Mr Trump was colluding with Russia, as the dossier maintained.

The dossier details an alleged meeting between Russian officials and ex-Trump lawyer Michael Cohen in Prague before the election.

But the special counsel's report suggests Mr Cohen did not travel to the Czech capital.

The dossier also claimed most infamously that Mr Trump cavorted with sex workers in Moscow in 2013, though Mueller noted that claim, too, was "unverified".

BBC News was able to confirm one claim in the dossier that a Russian diplomat in Washington DC was a spy.