Mr Gubarev and his company Webzilla took legal action against Mr Steele and Orbis Business Intelligence, a corporate intelligence company co-founded by Mr Steele, after the dossier was published by BuzzFeed in January 2017.
He is claiming “very substantial damages” over allegations that he and his companies “were knowingly using their servers for cyber-crime”, which Mr Gubarev denies.
At a trial in July, Mr Gubarev’s barrister Andrew Caldecott QC argued that the “sensational” allegations in the Steele dossier created “an obvious likelihood of serious financial loss”.
He also argued that Mr Steele and Orbis were “responsible in law” for BuzzFeed’s publication of the Steele dossier, which he said was produced for the “Democratic National Committee and/or Hillary Clinton’s presidential election campaign”.
But Gavin Millar QC, representing Mr Steele, said the publication by BuzzFeed of the “confidential” dossier was “unauthorised”.
Giving evidence at the Royal Courts of Justice, Mr Steele said the information he had gathered suggested Mr Trump and his administration might pose a “serious risk” in relation to the receipt of intelligence involving Russia, and that he was “duty bound” to provide it to the authorities.
Mr Steele also told the court of his “shock and horror” at what he described as BuzzFeed’s “reckless” and “irresponsible” publication of the dossier.
The court’s ruling is due to be delivered remotely at 10.30am on Friday.