US senators have asked Russian oligarch Oleg Deripaska if he worked with the former MI6 officer behind the the dossier containing salacious allegations about Donald Trump.

Senator Chuck Grassley, who chairs the Senate Committee on the Judiciary, has written to Mr Deripaska's London-based lawyers to formally ask him if his client has worked with Christopher Steele.

The allegation were made in a series of private memos before the 2016 election and included the claim Mr Trump asked prostitutes to perform a lurid act while in Russia.

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US senators have asked a lawyer representing Oleg Deripaska (left) about the oligarch's relationship with Christopher Steele (right)

The President branded the dossier 'a pile of garbage' calling it a 'tainted' document that the FBI must not use as a basis for investigation.

In his letter to lawyer Paul Hauser, which was sent last month, Mr Grassley asks whether there are any links between him, Mr Deripaska and Mr Steele, the Telegraph reported.

He asked in the letter: 'Have you ever hired or otherwise worked with Mr. Christopher Steele?' or his companies and continues 'if so, when, and what was the nature of the arrangement?'.

Sen. Chuck Grassley has demanded to know from London-based lawyer Paul Hauser if Deripaska has employed Steele

'Is it the case that Mr Steele, through you, works or has worked on behalf of Mr Deripaska or businesses associated with him? If so, when has such work occurred?,' Sen. Grassley wrote.

'Are you otherwise aware of any business or financial relationships between Mr Steele and Russian government officials, Russian oligarchs, or Russian businesses?'

Mr Steele's 35-page dossier claims that Russia collected a file of compromising information on Trump including sex tapes with which to blackmail him.

He was initially hired by a US political research firm FusionGPS by Republicans who wanted to stop Trump's campaign to win the Republican nomination for the 2016 election.

But he was kept on by Fusion GPS even after Trump won the nomination and his information was passed to the Democratic Party.

Mr Steele, 53, runs two companies Orbis Business International and Orbis Intelligence Limited with business partner Charles Burrows, 59.

Orbis Intelligence saw its retained profits more than double from £199,000 ($280,000) to £441,089 ($620,000) last year.

At the beginning of this month the House Intelligence Committee released a Republican memo which alleged the FBI relied on an unverified dossier compiled by Mr Steele that they used to look into any connections between Trump's campaign and Russia.

President Trump, who approved the release of the GOP memo, has so far declined to do the same with a second Democratic memo in its current form on the grounds it contains sensitive information.

A spokeswoman for Mr Hauser's law firm, Bryan Cave, declined to comment.