Christopher Steele, who runs a London-based intelligence firm, left his home in Berkshire, south-east England, earlier this weekand had asked his neighbour to look after his cats, BBC reports.
Steele, 52, has been widely named as the author of the memo, published in some US media containing extensive allegations about Trump's personal life and his campaign's relationship with the Russian state.
Trump has dismissed the claims as "fake news" and "phoney stuff".
According to BBC, Steelewas reportedly "in fear of his life", having spoken out about potential Russian involvement in Trump's election.
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Members of the intelligence community described the spook as "extremely, highly regarded" and was seen as "competent".
Steele, who was initially named in the US, is a former member of the Secret Intelligence Service (MI6) and a director of Orbis Business Intelligence, which describes itself as a leading corporate intelligence consultancy.
Founded in 2009 by former British intelligence professionals, the company based in Grosvenor Gardens in central London has a "global network" of experts andsays it offers "strategic advice" as well as mounting "intelligence- gathering operations" and cross-border investigations.
He is reported to have supplied the FBI with information on allegations of corruption at FIFA, football's world governing body.
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