It comes as the AIM-listed group warned of “material instances of deception” by a former employee between 2009 and 2011.
Last year, the Wall Street Journal alleged that an ex-employee requested a bribe for £64.7mln to complete a deal.
It said the former executive “met an employee of a New York-based architecture firm and requested 3.5% of the contract value be paid to an United Arab Emirates official helping fund the project”.
Sweett Group today said there have been further talks with the Serious Fraud Office in the UK and America’s Department of Justice, but no proceedings have so far been issued by either body, it added.
It has commissioned an investigation into the claims, which will be carried out by legal firm Mayer Brown LLP.
“Whilst this investigation is at an early stage and is ongoing, to date still no conclusive evidence to support the original allegation has been found,” the company said in a statement.
“However, evidence has come to light that suggests that material instances of deception may have been perpetrated by a former employee or employees of the group during the period 2009 - 2011.”
These findings are being investigated further, the group revealed.
Shares in the group slumped 25% to 38p on the news.