An ex-MI6 officer who is believed to have prepared memos claiming Russia has compromising material on US President-elect Donald Trump is now in hiding, the BBC understands.
Christopher Steele, who runs a London-based intelligence firm, is believed to have left his home this week.
The memos contain unsubstantiated claims that Russian security officials have compromising material on Mr Trump.
The US president-elect said the claims were "fake news" and "phoney stuff".
Mr Steele has been widely named as the author of a series of memos - which have been published as a dossier in some US media - containing extensive allegations about Mr Trump's personal life and his campaign's relationship with the Russian state.
Among the allegations are that Moscow has a video recording of Mr Trump with prostitutes and damaging information about his business activities.
BBC News correspondent Paul Wood said he understood Mr Steele left his home on either Tuesday or Wednesday, before he was publicly named, and was now "in hiding".
He said he had been told Mr Steele, a father of four, had asked his neighbour to look after his three cats.
Our correspondent said he had been shown the memos about Mr Trump in October last year, when he was told Mr Steele was "in fear of his life", having spoken out about potential Russian involvement in Mr Trump's election.
He said he had been told by members of the intelligence community that Mr Steele was "extremely highly regarded" and was thought of as "competent".
The central allegation made in the memos was that Mr Trump was "vulnerable to blackmail", he added.
Mr Steele has not responded to the BBC's request for a comment about the revelations about Mr Trump.
By Paul Wood, BBC news correspondent
Claims about a Russian blackmail tape were made in one of a series of reports written by a former British intelligence agent.
As a member of MI6, he had been posted to the UK's embassy in Moscow and now runs a consultancy giving advice on doing business in Russia. He spoke to a number of his old contacts in the FSB, the successor to the KGB, paying some of them for information.
They told him that Mr Trump had been filmed with a group of prostitutes in the presidential suite of Moscow's Ritz-Carlton hotel.
I know this because the Washington political research company that commissioned his report showed it to me during the final week of the election campaign.
The BBC decided not to use it then, for the very good reason that without seeing the tape - if it exists - we could not know if the claims were true. The detail of the allegations were certainly lurid.
The entire series of reports has now been posted by BuzzFeed.
Mr Steele, who was initially named in the US, is believed to be a former member of the Secret Intelligence Service - MI6 - and is a director of Orbis Business Intelligence - which describes itself as a leading corporate intelligence consultancy.
Founded in 2009 by former British intelligence professionals, the firm - based in Grosvenor Gardens, central London - has a "global network" of experts and "prominent business figures", according to its website.
Orbis says it offers "strategic advice", as well as mounting "intelligence-gathering operations" and cross-border investigations.
Christopher Burrows, who is listed as a co-director of Orbis alongside Mr Steele, refused to confirm or deny whether the firm had produced the reports on Mr Trump.
BBC security correspondent Frank Gardner said Mr Steele had previously been an intelligence officer - rather than agent - in MI6, who would have run a team of agents as an intelligence gatherer.
However, as Mr Steele was now working in the private sector, our correspondent said, there was "probably a fair bit of money involved" in the commissioning of the reports.
He said there was no evidence to substantiate the allegations and it was still possible the dossier had been based on what "people had said" about Mr Trump "without any proof".
Mr Steele reportedly spent years under diplomatic cover working for MI6 in Russia and France, as well as at the Foreign Office in London.
He is reported to have supplied the US Federal Bureau of Investigation (FBI) with information on allegations of corruption at Fifa, football's world governing body.
Prime Minister Theresa May's official spokeswoman said she would not comment on allegations against Mr Trump, other than to say those allegedly involved in producing the document were "former employees" of the government.
Asked whether Downing Street knew if any current British agents had any role in the dossier, she said she had seen nothing in any of the reports to suggest so.
Labour MP Mary Creagh called for Foreign Secretary Boris Johnson to tell parliament what the UK government knew about the dossier.
Meanwhile, Marina Litvinenko, the widow of murdered Alexander Litvinenko, who was critical of Russia's leader Vladmir Putin, said Mr Steele had been put in danger by collecting such information.
She told the BBC: "I believe it is very dangerous, particularly after death of my husband, because when you just approach very specific information, particularly when this information very close to very powerful people, you might be in this line and you just easily might be killed."
The 35-page dossier on Mr Trump - which is believed to have been commissioned initially by Republicans opposed to Mr Trump - has been circulating in Washington for some time.
Media organisations, uncertain of its credibility, initially held back from publication. However, the entire series of reports has now been posted online, with Mr Steele named as the author.
Intelligence agencies considered the claims relevant enough to brief both Mr Trump and President Obama last week.
But the allegations have not been independently substantiated or verified and some details have been challenged as incorrect by those who are mentioned.
Mr Trump himself was briefed about the existence of the allegations by the US intelligence community last week but has since described them as fake news, accusing the US intelligence services of leaking the dossier.