The official register of MPs interests outside parliament  is not working as it should, experts have said, following an investigation by current affairs programme Nieuwsuur.

The register, which was set up to promote transparency about the source of MPs’ extra income and other activities, is incomplete, with MPs apparently unwilling or unable to provide the necessary information, Nieuwsuur said.

MPs are allowed to have jobs outside their work as an MP but one in three of the 150 MPs had failed to register these interests correctly, the investigation found. This makes it difficult to gauge how independent actually MPs are, experts told the programme.

Some 41 MPs corrected their entry following queries from Nieuwsuur while 24 MPs amended them but not completely. Some 28 MPs did not react to the queries at all.

Among the problems the researchers found were MPs who failed to declare their income from real estate investments or from other political jobs such as sitting on a local or provincial council. Others did not include company directorships in the register, or shareholdings.

Experts said MPs are using a very narrow interpretation of the rules when it comes to describing their other activities. “I would have expected less prevarication and more transparency, particularly from an MP,” integrity expert Rob van Eijbergen said.

“I am flabbergasted,” Jan Kleijssen, former director of the anti-corruption Council of Europe watchdog Greco, which had recommended stricter adherence to the register as early as 2013, said.  “This goes beyond what Greco stands for and what we recommend, that is, a detailed registration of every outside activity,” he said.

Greco said last October the Netherlands still has to do more to tackle potential government corruption.

Constitutional law professor Wim Voermans said the lack of transparency from MPs could undermine trust in parliament as an institution.

A letter explaining the rules has since been sent to MPs but that too was “far too vague”, the experts said.