The Serious Fraud Office has been criticised for keeping the details of its new boss under wraps after it announced that a temporary chief will hold the fort for an unknown length of time.

The search for a successor to outgoing director David Green has been going on for months, with sources saying SFO general counsel Alun Milford and two external candidates had made the final round of interviews.

The Attorney General's office said on Tuesday that a person had now been chosen but is "undertaking the final stages of the appointment process and managing their exit from their current position" and so won't start the role until later this year.

In the meantime it said that Mark Thompson, who is currently chief operating officer at the white-collar crime authority, will fill in for Mr Green as interim director once he ends his six-year tenure on April 20.

Whitehall insiders had initially tipped prominent barristers Sascha Wass QC and Max Hill QC as key contenders, but sources say neither is the mystery candidate. Although the position is a prestigious one, City legal recruiters said it can be hard to fill due to conflicts of interest and a low salary compared to what lawyers can earn in private practice.

Alison Geary, who acts for individuals involved in some of the SFO's investigations, argued that the lack of detail in the Attorney General's announcement "sparks more questions than it answers".