The Guardian has been prevented from reporting parliamentary proceedings on legal grounds which appear to call into question privileges guaranteeing free speech established under the 1688 Bill of Rights.

Today’s published Commons order papers contain a question to be answered by a minister later this week. The Guardian is prevented from identifying the MP who has asked the question, what the question is, which minister might answer it, or where the question is to be found.

The Guardian is also forbidden from telling its readers why the paper is prevented – for the first time in memory – from reporting parliament. Legal obstacles, which cannot be identified, involve proceedings, which cannot be mentioned, on behalf of a client who must remain secret.

Ian Dale and others speculate that the underlying story is a 2006 toxic oil waste dump near Abidjan, the capital of Côte d’Ivoire. British oil trader Trafigura had publicly claimed the waste was harmless, but recently offered to pay £100m damages.