A retrial has been ordered after the trial of Achilleas Kallakis ? accused of using forged paperwork to trick banks into lending ?750m ? collapsed after four months

The trial of Achilleas Kallakis, the onetime Mayfair property tycoon accused of using forged paperwork to trick banks, including HBOS and Allied Irish Bank, into lending ?750m, has collapsed after four months. A retrial has been ordered.

A judge at Southwark crown court told the jury that Kallakis's co-defendant Alex Williams had been admitted to hospital and that medical reports suggested he was unlikely to be fit enough to return to the court for two to three weeks.

Judge Andrew Goymer told the jurors, who had been hearing evidence since 29 September last year: "This case has been bedevilled by a whole series of delays. I don't need to spell them out to all of you ? five weeks of [new evidence] disclosure, [then] there was various things that have happened since."

One juror had already been discharged some weeks earlier because of work commitments. The remaining 11 men and women were let go on Monday, after the judge thanked them and told them they had been placed under "an intolerable burden". He said they would now be exempted from having to serve on another jury for life.

Kallakis, the nephew of Greek shipping tycoon Pantelis "Lou" Kollakis, and Alex Williams, a friend from university, have pleaded not guilty to 21 counts, including conspiracy to defraud, relating to alleged fraudulently obtained loans.

The jury was told the men had changed their names since pleading guilty, at the same court 16 years ago, to a forgery conspiracy relating to a scam to sell bogus honorary titles to unsuspecting Americans. Kallakis had been convicted under the name Kollakis while Williams had been convicted under the name Lewis.

Kallakis had spent four days in the witness box giving evidence. He told the court he had no idea that forged documents had been prepared for the banks to obtain loans between 2003 and 2008. He had been "dumbstruck" when AIB confronted him with allegations of fraud.

Kallakis, who operated out of offices in Mayfair as well as a yacht in Monaco, described AIB as having been extremely eager to do business with him. "They are anxious to get over to me that they were in the market, were awash with money, were anxious to lend. I think AIB was relatively new to the property finance world and were very, very anxious to build up their loan book."

Central to the Serious Fraud Office's case against Kallakis is that he sent to the banks forged rental guarantees purporting to be provided by the Hong Kong-listed company Sun Hung Kai Properties. These allegedly bogus over-riding lease commitments from SHKP transformed bank valuations of properties targeted by Kallakis for acquisition, allowing him to access much larger loans.

The court earlier heard evidence from SHKP executives denying they had provided any guarantees. Kallakis claimed in court that he had been introduced to SHKP by a middleman broker, who had stipulated the two sides must not meet one another. Asked if he thought it unusual that the broker had often requested to be paid in cash, he said he did not.

Earlier in the trial, Victor Temple QC, prosecuting counsel for the SFO, characterised Kallakis as the fraud frontman, saying he was "confident, assertive and, on occasion, arrogant". Meanwhile, Williams, he claimed, was "self-effacing, quiet and unassuming ? [and was] a versatile and prolific forger".

Witnesses who worked at AIB at the time admitted in court that they had accepted hospitality from Kallakis, including a trip to the World Cup final in Germany in 2006 and to the Monaco Grand Prix. Some also told the court they had accepted a three-night trip to Mauritius, offered by Kallakis as a "thank you" for their work on one property deal.