At a time of momentous constitutional change, Britain needs an effective political opposition. It has instead an insular and increasingly intolerant one. A government of depleted authority is escaping proper scrutiny because Labour is busy transforming itself from a reformist party of the centre left into a narrow and extremist sect.

The evidence is widespread. Ahead of next year’s municipal elections, activists loyal to Jeremy Corbyn are purging the party of centrist councillors for showing insufficient ideological purity. There is little point in appealing to Mr Corbyn to stand by those who defend Labour’s traditional values, because he was elected leader precisely for the purpose of abandoning them.

Across Labour’s urban areas of strength, councillors are being deselected or are stepping down to escape intolerable