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The company which the Electoral Commission suspects may have been behind Britain's biggest political donation has no registered address - meaning it has failed to fulfil a key legal requirement - the BBC has found.

Rock Holdings is a firm registered in the Isle of Man and controlled by Leave.EU founder Arron Banks.

The Electoral Commission's ongoing investigation of Leave.EU funds was referred to the National Crime Agency.

Mr Banks denies any wrongdoing.

But BBC Newsnight has found that Rock Holdings has no official registered address on the Isle of Man and that it is therefore "in default", meaning it is not compliant with company law on the island.

Meanwhile one of Mr Banks's companies has obstructed the BBC's attempt at seeing Rock Holdings' accounts - despite formally agreeing with Companies House that it would ensure the documents would be made publicly available.

Mr Banks is a multimillionaire insurance mogul behind the Go Skippy car insurance brand.

He loaned £8m to the Leave.EU campaign before the 2016 EU referendum and says the money came from his company, Rock Services, based at his office in Catbrain Lane, Bristol.

The Electoral Commission suspects the money may have come from another Banks entity, Rock Holdings, based on the Isle of Man.

They, along with the National Crime Agency, are examining whether "impermissible money" may have been used to help fund the Leave campaign before the 2016 EU referendum.

The law on political money is simple. The source should be clear, transparent and British.

If the £8m came from Rock Holdings, then the money would be impermissible and Mr Banks would be in trouble.

Mr Banks says he is a British taxpayer and the money was generated in Britain.

Following the trail to Rock Holdings

For more than a year, accountant and investigative journalist Iain Campbell has been looking into Mr Banks's financial empire, all of which is private and some is held offshore in Gibraltar and the Isle of Man.

Mr Campbell noted that Rock Services has successfully qualified for a limited disclosure exemption under the Companies Act, allowing it to provide less information in its accounts.

But in order to gain this exemption, it had stated in its notes that the accounts of its parent company, Rock Holdings, were "publicly available" at Catbrain Lane.

So last month, Mr Campbell tested this out in person. He went to the Catbrain Lane base, only to be told that they were not available "that day".

Since then, lawyers for Rock Services, Browne Jacobson, have written to the accountant setting out a series of restrictions on how, when and where the "publicly available" accounts could be viewed and how their contents could be recorded. Mr Campbell told Newsnight: "This is completely unacceptable."

Newsnight made its own trip to Catbrain Lane this week but when reporter John Sweeney requested an opportunity to inspect the "publicly available" accounts of Rock Holdings, he was asked to leave and escorted out of reception.

Image caption Newsnight's John Sweeney went to Catbrain Lane to inspect the "publicly available" accounts, but was asked to leave.

Browne Jacobson later emailed the BBC with an explanation - referring to a "risk assessment" by Mr Banks's company on "safety and security" - which, in effect, denied the BBC sight of the accounts at Catbrain Lane.

Where to find Rock Holdings?

At the time of the referendum, Rock Holdings' address had been registered at Murdoch Chambers, South Quay, Douglas, Isle of Man.

Newsnight visited island this week and its first stop was to Murdoch Chambers, which now appears to be an accommodation address, facing a gas showroom overspill car park. The door was locked and no-one answered.

The Newsnight team then moved on to 18 Athol Street, also in Douglas. This was the address, albeit "care of Greystone", that Rock Holdings filed with the Isle of Man registry in November this year.

But when Newsnight dropped by, it was told by Greystone that Rock Holdings "was neither approved nor agreed by or known to any director or officer of the company or ourselves".

This means that the company that the Electoral Commission suspects may be the source of the biggest donation in British political history is a closed book, in a secrecy jurisdiction with no official address and when you try to look at its accounts you cannot.

Any further questions to answer?

Newsnight has separately investigated Ural Properties. This company name appeared in an internal email chain sent by Mr Banks's team to the BBC, to prove his innocence of the charge that the money might have come from Russia.

Mr Banks faced controversy after his three meetings with the Russian ambassador became public knowledge and over whether he travelled to Russia in 2016, the year of the referendum. He previously told Newsnight: "No money came from the men with snow on their boots."

One internal email in the documents sent to the BBC contained the comment: "Redact the reference for Ural Properties and any references which include sensitive info e.g. the account numbers that the money was sent from."

An online website, Source Material, obtained the accounts of Ural Properties in Gibraltar and passed them on to Newsnight.

Two nominee companies have owned Ural Properties and the first is linked to a known associate of Mr Banks, Alan Kentish. Last year Mr Kentish was arrested by the Gibraltar authorities for alleged non-disclosure of information. No charges followed.

The company owns two flats in Gunwharf, Portsmouth, overlooking the sea lane into the Royal Navy base.

Newsnight asked Mr Banks specific questions about Rock Holdings, the fact that it was in default and its lack of a registered address. Mr Banks was also asked about Ural Properties. He made no substantive reply.

However, he did provide the following statement: "This is the seventh programme designed to smear me and discredit Brexit. I have repeatedly stated that I am a UK taxpayer and the funds were from UK. The NCA have had 'full disclosure' of the relevant bank accounts and an explanation of the where the money came from. We will be doing a victory lap of the TV studios once this matter is resolved and look forward to being interviewed by John 'the Trot' Sweeney or the ever 'professional' Kirsty or Emily."

You can watch Newsnight on BBC 2 weekdays 22:30 or on iPlayer. Subscribe to the programme on YouTube or follow them on Twitter.