Its registered office is a terraced house in the town of Grays in Essex, occupied by Tanja Pazarcik, who works for Insolution Service, an agency that helps people set up companies. It uses the address for a number of its clients. Pazarcik says she forwards United's mail to an address in Austria but otherwise knows little about the firm.

United Capital's accounts filed in late October showed the company had 14 pounds in cash and is dormant. The transaction price for Doverie has not been revealed. In a July 15 statement the seller, Vienna Insurance Group AG, said only it had agreed to sell its 92.58 percent of Pension Insurance Company Doverie to United Capital PLC.

Such gaps have raised questions in Bulgaria about United Capital's intentions for Doverie, which with almost 1.8 billion levs (784.7 million pounds) under management and more than 1.25 million contributors is an important pension provider in Europe's poorest country.

In Britain, transparency campaigners have latched onto the controversy to renew their calls for tougher disclosure rules.

A Reuters examination of company documents and interviews in Britain, Austria, Bulgaria, Russia and Hong Kong has traced the ownership of United Capital to a series of corporate entities.

Bulgaria's ruling Socialists have appealed to the country's financial regulator to halt the sale. The watchdog says it will seek more information about United Capital's owners, finances and how it proposes to fund the purchase when United Capital files its documents for approval of the transaction.

A spokesman for Vienna Insurance Group said it carried out an examination of all the offers it received from several parties and, after an evaluation, put together a shortlist.

"From there, the final decision was for United Capital," the spokesman said. He did not respond to requests for further comment on what Vienna Insurance knew about United Capital.

In an emailed statement on July 26, United Capital said it was a UK-based financial services "holding entity", which, along with affiliates, manages more than 2 billion euros and which has "participation" in insurance companies, real estate funds, banks and industrial assets in the EU, U.S. and Russia.

"There's nothing the slightest bit unusual about this purchase or selection process which was vetted (by an independent entity)," said U.S.-based lawyer Deborah Sturman, chairman of United Capital, in a telephone interview.

"United Capital is a perfectly normal U.K.-registered financial services holding company." When asked about concerns over the deal in Bulgaria, she said: "if people have a problem with it, I don't care."

"DUE DILIGENCE"

According to UK regulatory filings United Capital was incorporated in 2007 by a UK firm called Fletcher Kennedy, which creates offshore companies. Fletcher Kennedy's website says the firm can set up companies in offshore centres such as Belize, the British Virgin Islands, Gibraltar and the Seychelles. It also says it can set up companies in the UK in three hours for a fee of 208 pounds.

"You do not have to be a UK national or resident to be an officer of a UK company," the website states. "The company must also have a registered office within the United Kingdom. This is the official address of the company, it does not have to be a trading address, and the company does not have to maintain a presence at the address. We will provide you with an address to use as your registered office."

Director Charles Fletcher said the firm was instructed by an individual he declined to name to set up United Capital, and "the complex structure that you mention came in long after our short involvement with the company ended."

He added: "In 2007 there was no requirement for formation agents to carry out due diligence checks. Having said that, we would have obtained identity documents from the instructing client."

"Companies having a registered office at an address other than their trading address is a matter for the legislators."

Once United Capital was founded Fletcher Kennedy resigned as director and secretary soon afterwards, according to regulatory filings. A formation agent acting as the first director and then resigning was standard practice at the time, Charles Fletcher said.

United Capital's accounts for the year to May 2012, filed in October, show the firm had almost 2.9 million pounds in cash. Amended accounts filed later that month show it had 14 pounds in cash. Both show the company to be dormant.

Bulgarian politicians and contributors to the Doverie pension fund say they want more details about United Capital's owners.

In its emailed statement on July 26, United Capital said one of its significant investors was Sergei Mastyugin, a Russian banker. United Capital chairman Sturman said in an emailed response to Reuters on Aug 2 that Asia Trade Management, based in Hong Kong and whose principal owner is Mastyugin, had a stake of very close to 50 percent.

A spokeswoman for Mastyugin was not available to comment.

Based on regulatory filings in Hong Kong, the business activities and shareholders of Asia Trade Management are unclear. A representative for Asia Trade Management declined to comment but said a spokeswoman from Investbank - a Russian bank in which Mastyugin has an 18.3 percent stake and where he sits on the supervisory board - would respond. That spokeswoman was not able to comment, however.

United Capital's emailed statement on July 26 also said LJ Capital, the London-based merchant banking arm of privately held LJ Group, had increased its interest in the firm. Sturman said that LJ Capital had a stake of very close to 50 percent. LJ Capital declined to comment.

"THE RELEVANT PEOPLE"

UK regulatory filings show that United Capital has four directors. Three - Slobodan Ristic, Guenter Rohr, and Dr Heinz Russwurm - live in Vienna or nearby. The fourth director is Sturman.

Rohr has been a director since 2007. Ristic, Russwurm and Sturman were appointed on July 11 of this year, the filings show, four days before Vienna Insurance Group announced it would sell its stake in the Bulgarian fund to United.

Rohr, who advertises himself as running an IT and corporate consultancy firm, said he would pass on questions from Reuters to "the relevant people".

Russwurm, who is listed in directories as a tax accountant, could not be contacted for comment, nor could Ristic.

Plamen Dimitrov, the head of Bulgaria's largest trade union CITUB which has a 1 percent stake in Doverie, said his union wanted to know more about United Capital.

"The company has changed its majority owner several times, but the buyers were always well established companies and now we get this company (United Capital) that no one has heard of," he said.

Previous majority owners include the European Bank of Reconstruction and Development, Deutsche Bank and the Dutch Kardan Financial Services Group. Deutsche Bank and Kardan declined to comment. The EBRD did not immediately respond to a request for comment.

Daniela Petkova, head of the Doverie management board, said the uncertainty about its future ownership was damaging the fund: "I do not know now what the fair value of the company will be, as many people have already filed declarations to change their pension fund because of the lack of clarity. We are speaking about thousands."

A general shareholder meeting of Doverie on July 30, at which Deborah Sturman was present, did not provide any new information on the buyer, Dimitrov said. Another meeting is set for August 20.

(additional reporting by Mark Anderson in London, Nishant Kumar and Clare Baldwin in Hong Kong, Maria Kiselyova and Megan Davies in Moscow, Georgina Prodhan and Angelika Gruber in Vienna and Lauren Tara LaCapra and Brian Grow in New York; writing by Simon Robinson; editing by Janet McBride)

By Laurence Fletcher and Tsvetelia Tsolova