Economic affairs minister Henk Kamp is refusing to publish a concept report on the risks attached to drilling for shale gas in the Netherlands, says environmental group Milieudefensie.

The minister is also keeping correspondence between his department and the researchers about the report confidential, the organisation says.

The organisation thinks it likely the minister pressurised civil engineering company Witteveen + Bos to temper its conclusions. ‘We are curious as to why the concept versions are being kept secret,’ a Milieudefensie spokesman said.

Milieudefensie is trying to access the documents using freedom of information legislation.

Of the 125 documents the group wants to access, 110 remain confidential.

The final report, published in September, concluded the risks attached to extracting shale gas are manageable. It has also been criticised for simply assessing current literature and not looking at the actual situation in the Netherlands.