The Communication Workers’ Union has said that the current round of industrial action has caused a back-up of mail “bigger than at the height of the national strike of 2007″. Twenty million items are said to be awaiting delivery in London alone, and delivery delays of over a week are being reported. Royal Mail has said that the figure is nine million nationwide, of which two-thirds is in the capital.

The union has now begun ballotting its members on national strike action. Royal Mail has called the strike “wholly irresponsible”.