Two friends have been convicted of conspiring to defraud banks of millions of pounds during a sophisticated five-year property and luxury yacht scam, the UK Serious Fraud Office said.

Achilleas Kallakis and Alexander Williams tried to swindle AIB out of £740m sterling and Bank of Scotland of £22m between 2003 and 2008.

During the trial, the jury was told that Kallakis used the proceeds of his fraud to fund the lifestyle of the super-rich.

He had a fleet of chauffeur-driven Bentleys, a private plane, a private helicopter and a luxury yacht moored in Monaco harbour.

The pair were convicted of two counts of conspiring to defraud the banks after a four-month trial. They will be sentenced tomorrow.

The men, from London, used forged documents as part of their plot to use AIB money to buy properties and BOS cash to convert an ex-passenger ferry into a super yacht, the court heard. 

The loss to AIB was more than £56m and £4.8m to BOS.

The two men, who both used various aliases, operated from London business premises in Carlos Place, Mayfair, where Kallakis, a one-time travel agent, masqueraded as a legitimate property tycoon and Williams a financial consultant.

The SFO said the borrowing represented £740m for a 16-property portfolio and a £22m loan agreement for the yacht conversion.

Kallakis and Williams operated out of their office as the Pacific Group of Companies and defrauded AIB by using the forged or false documents or claims in order to obtain substantial loans to finance the purchase of what was mostly a commercial property portfolio.

The transactions were structured in a way that the bank loans exceeded the purchase price of the properties using a guarantee from a well-established Hong Kong company, Sun Hung Kai Properties Limited (SHKP), that knew nothing about the guarantees because they were forged by the men.

Kallakis and Williams were able to maintain the deception over five years through skilful forgeries and manipulation of AIB.

When in 2007 the bank requested a meeting with a representative of SHKP, Kallakis and Williams set up a meeting at their offices with an individual who falsely presented himself as being from the SHKP Treasury Department.

The loans advanced by AIB represented £60m in excess of the cost of the properties. But by August 2008, alarm bells rang at AIB when they learnt that Kallakis had a previous fraud conviction in the name of Stefanos Michalis Kollakis and had subsequently changed his name. They contacted SHKP and found they knew nothing about the guarantees.

During 2007 and 2008, in another fraud following similar lines to the property loans fraud, the Bank of Scotland agreed a loan of £24m which was wanted, claimed Kallakis, for the conversion of a former passenger ferry into a super-yacht for his personal use.

He provided the bank with a worthless guarantee from the Oregon Finance Corporation and forged documents as the bank sought to carry out checks. The ferry upon which the loan had been secured turned out not only to have no value but instead was a significant net liability as it was contaminated with asbestos and no longer water tight.

By the time suspicions were raised, the bank had advanced a proportion of the loan - £4.7m. Following the alarm being raised, police searched the defendants' London residences, and their Mayfair office was searched in March 2009, and they were charged in February 2010.

Williams also had previous convictions, under his then surname of Martin Lewis, for four offences of deception in relation to obtaining false passports. His forgery skills contributed to the execution of the fraud against the banks, the SFO added.

A third person, Swiss lawyer and businessman Michael Becker, is also alleged to have conspired with the defendants. The SFO claimed he was closely involved in the fraud and was director of companies presented to the banks in the loan agreements as "borrowing companies". As he is a Swiss national, he has not been charged due to his absence from UK jurisdiction.