Six people have been arrested in Spain on suspicion of a 600 million US dollar (£426 million) fraud on the London stock market, police said today.
Spanish police said the arrests in Madrid, Barcelona and Elche were linked to an alleged fraud in 2003.
An investigation was launched in 2005, according to the police statement.
The arrests were linked to a Serious Fraud Office investigation into a company called Langbar International (formerly Crown Corporation Ltd), which is listed on the AIM section of the London Stock Exchange.
It relates to accusations of artificially increasing the value of the firm's shares.
British investigators from the Serious Fraud Office (SFO), with the help of Spanish authorities, carried out searches of properties in Spain last week.
The SFO said five of the six men arrested were Spanish, while one was Argentinian.
Their ages ranged from 56 to 76.
An SFO spokesman said: "Search and arrest operations were undertaken by the Spanish national police together with the economic crime and money laundering group, the superior chief police of Catalunia with the support of the fraud unit Udes Madrid and the local police of Elche (Alicante) alongside SFO investigators of the SFO and City of London Police last week at four residences and two business premises in Madrid, Barcelona and Alicante.
"Six people were arrested and interviewed in relation to the SFO investigation into Langbar International.
"As part of the ongoing investigation into certain individuals previously associated with Crown Corporation Ltd, now Langbar International under new management, the SFO obtained collaboration from the Cuerpo Nacional de Policia and a mutual legal assistance procedure to undertake searches, arrests and interviews.
"The operations were co-ordinated by Crime Court 24 of Barcelona.
"The six persons arrested were male and their ages ranged from 56 to 76 years.
"They are Spanish nationals, except for one who is an Argentinian national."
The Spanish national police said the men were arrested on January 21.
The SFO launched an investigation in relation to Langbar in November 2005, after the company issued a statement to the stock market announcing it could not "establish the existence of, nor verify its entitlement to" around £370 million held in Dutch and Brazilian banks.
The company had already requested the suspension of its shares.
Later that year, the company announced a new board.
No-one from Langbar was available to comment today but a statement on the company's website reads: "The present board of Langbar International Limited is determined to discover exactly what has transpired concerning company funds and to pursue their recovery.
"Our aim is to restore value for shareholders to the maximum that can be achieved.
"Under the instruction of Jones Day Solicitors, Risk Profiling Limited is undertaking extensive inquiries in a number of jurisdictions to fully identify and recover Langbar's missing funds.
"These investigations are well advanced and they are maintaining a close dialogue with UK investigating authorities."
No-one from the London Stock Exchange (LSE) was able to confirm details of the investigation.
Langbar's former nominated adviser, Nabarro Wells - which was tasked with checking the company complied with AIM rules - was consequently fined and publicly censured in 2007 for breaching Stock Exchange rules.
In a statement on its website, the Spanish national police said the allegations were that: "Through complex operations and falsifications the suspects managed to make the value of a company's shares in the Stock Exchange increase without the deposits being there to guarantee them. The shares were then fraudulently sold."
It said the alleged fraud began in 2003 when a company was entered on the AIM section of the stock market.
According to the police, investigators allege the company's entrance to the market and sale of shares were "obtained through fraud and that there were various false announcements in the specialist media in London to generate interest in the shares of the company".
The announcements were allegedly placed by the main suspects who afterwards sold the shares.