Details of ex-Serious Fraud Office (SFO) director David Green QC's move to Slaughter and May have been revealed in a government letter that sets out the conditions of his new job, as well revealing details of the magic circle firm's role on the multimillion-pound Tchenguiz brothers lawsuit brought against the SFO in 2013.

The letter was published today (12 September) by the Advisory Committee on Business Appointments (ACoBA), which vets new roles taken up by former ministers, senior civil servants and other Crown servants, to avoid conflicts of interest.

Acknowledging that Green, as head of the SFO was "responsible for decisions which benefited Slaughters", the letter states that the committee recognised there could be concerns over a "high-profile public office holder moving from a pivotal role within the criminal justice system to a private law firm operating in the same area".

As such, the ACoBA has imposed a number of conditions on his move. These include that Green: should not advise on any matters relating to investigations conducted by the SFO during his tenure as director; he should not draw on any privileged information available to him from his time at the SFO; that he should not directly engage with the body on behalf of Slaughters or its clients; or be personally involved in lobbying government or the SFO for two years from his last day at the department.

The letter also contains details of Green's official dealings with Slaughters during his tenure at the SFO – in particular the multimillion-pound claims resulting from its failed investigation into entrepreneur brothers Robert and Vincent Tchenguiz.

The letter states that Slaughters was paid fees of "circa £15m" for representing the SFO in the legal battle, which saw the body sued for damages by the brothers in January 2013. The case, in which the SFO admitted errors in the evidence it used to obtain search warrants against the pair, was settled for £4.5m in 2014.

The letter reveals that four City firms were approached to advise the SFO on the lawsuit, but Slaughters was selected as it had no conflicts to prevent it from acting and "offered a lower rate". It adds that Green played no role in selecting Slaughters.

Other SFO cases referred to in the letter include the investigation into Rolls-Royce, which concluded with a groundbreaking £497.25m deferred prosecution agreement last year.

Green said he attended one meeting with Slaughters and the CEO and chair of Rolls-Royce to "reset" negotiations during the case, and that he had no other dealings with Slaughters with regards to other clients of the firm under investigation by the SFO.

Slaughters was also required by the ACoBA to provide evidence of how it would deal with any potential conflicts presented by Green's hire, stating that matters relating to the SFO would be put behind an "information barrier", meaning that he would be unable to access information on the cases from the moment he joins the firm.

Green's move to Slaughters was confirmed yesterday (11 September), ending months of speculation. He will join the magic circle firm as senior consultant on 22 October, six months after he left the SFO in April.

The letter states that Green was approached about joining Slaughters by the firm's global investigations group co-heads, Richard Swallow and Jonathan Cotton.