The Serious Fraud Office (SFO) has settled civil damages claims brought by property tycoon Robert Tchenguiz and his businesses for £1.5m, days after settling with brother Vincent for £3m.

The amount, settled out of court, will be paid to Robert Tchenguiz within the next 14 days. The agreement means that a scheduled trial in October will not take place.

SFO director David Green said: "I am pleased that we have been able to resolve this final outstanding matter, without the need for a costly trial. As I said when Mr Vincent Tchenguiz accepted our offer last week, the SFO deeply regrets the errors for which we were criticised by the High Court in July 2012.

"On behalf of the SFO, I also apologise to Robert Tchenguiz for what happened to him. I reiterate that the SFO has changed a great deal since March 2011, and I am determined that the mistakes made over three years ago will not be repeated."

Earlier this month, Robert Tchenguiz replaced counsel from Shearman & Sterling, led by partner Jo Rickard, with a team from Stephenson Harwood, led by commercial litigator Sean Jeffrey, who was already advising Vincent.

Slaughter and May has been representing the SFO since February last year on defending claims totalling as much as £300m. The claims followed the agency's failed investigation into the Tchenguiz brothers (pictured). Slaughters' team is led by disputes partner Jonathan Cotton.

Slaughters instructed Blackstone Chambers' James Eadie QC, Pushpinder Saini QC and James Segan, as well as One Essex Court's Charles Graham QC, Simon Colton and Patricia Burns and 6KBW College Hill's Katherine Hardcastle.

Vincent Tchenguiz agreed to a £3m settlement earlier this week. As part of the agreement the SFO will also pay for legal costs incurred during the long-running dispute, including another £3m towards the final costs figure, within the next month.

Both Robert and Vincent Tchenguiz are now preparing to sue accountancy firm Grant Thornton and other third parties for their part in the investigation, with reports saying claims could reach hundreds of millions of pounds.

Robert Tchenguiz said of his settlement: "I am satisfied that the taxpayer should not bear the full financial pain of the SFO, and its former director's, misguided actions. I believe that the ultimate blame lies elsewhere.

"I do not believe that, without external influences, the SFO would ever have had any possible cause to arrest me for suspected wrongdoing. It is now quite evident that third parties played a major part in this hugely damaging farce.

"Having resolved my issues with the SFO, I now intend to join my brother in pursuing those who I believe to be both responsible and liable for the devastation that has been caused."

A spokesperson for Grant Thornton said: "Grant Thornton has acted appropriately, and in accordance with its professional responsibilities and legal obligations, throughout. Any disclosure made has been accurate and in accordance with those professional and legal obligations.

"It is the responsibility of the investigating agency to review and interpret any information provided to it, and to act as it sees fit. Grant Thornton did not act as advisor to the Serious Fraud Office."

The fraud agency conducted a long-running investigation into the Tchenguiz brothers for suspected impropriety in relation to the failure of Icelandic bank Kaupthing in 2008.

The pair were arrested in March 2011 in a highly-profile dawn raid, but the SFO later conceded there were errors in the evidence it used to obtain search warrants against the brothers.

The High Court quashed the search warrants in 2012 after the brothers challenged the SFO through a judicial review, ruling that the agency's handling of the investigation was unlawful.