The number of letterbox, or shell, companies registered in the Netherlands has fallen by almost one third over the past five years, the Financieele Dagblad said on Tuesday.

Tougher rules for trust offices and more stringent supervision by the central bank are among the reasons for the decline, the paper said.

In 2014, there were some 25,000 shell companies registered in the Netherlands, according to central bank figures. By the end of 2018, the total had gone down to 17,000.

Customers are leaving trust offices because of increased demands and the rising cost of compliance and transaction monitoring, Holland Quaestor, the trust sector lobby group, told the paper.  Some are liquidated while some opt to go it alone, where they remain outside central bank supervision, the organisation said.

The FD reported on the figures in connection with its investigation into the financial affairs of Isabel dos Santos, the daughter of the former president of Angola.

According to investigative journalists platform ICIJ, Dos Santos has become the richest woman in Africa by exploiting family ties, shell companies and being involved in unscrupulous deals.

Some of her millions were managed via Dutch limited companies (BVs), three of which were involved in controversial transactions, the paper said.