Concern about mass migration is a “terrorist ideology” that requires intervention by the Government’s anti-radicalisation Prevent programme, according to official documents.

An online training course hosted on the Government’s website for Prevent lists “cultural nationalism” as a belief that could lead to an individual being referred to the deradicalisation scheme.

This encompasses a conviction that “Western culture is under threat from mass migration and a lack of integration by certain ethnic and cultural groups”, staff taking the course are told.

It is the latest example of officials, police and the courts appearing to restrict free speech.

In recent weeks, The Telegraph has disclosed that a pensioner was wrongly arrested for warning of anti-Semitism on Twitter, as well as highlighting the case of a man who was given a criminal sentence for burning the Koran.

The Prevent programme was the subject of a damning report two years ago and failed to stop Axel Rudakubana, the Southport killer, who was referred to it on three separate occasions.

Free speech critics warned that the definition of “cultural nationalism” was too widely drawn and could, in theory, mean that even Sir Keir Starmer and Robert Jenrick, the shadow justice secretary, would fall foul for their past comments.