I just got my weekly TfL 'Weekend travel information' newsletter, which
contains this item of information:

From Friday 26 until Monday 29 March, due to staff shortages, there will be
short notice closures on the Hammersmith & City, Circle and Metropolitan
lines.

The Circle and Hammersmith & City lines will be disrupted on the following
days:
On Friday 26 March, services will be reduced from 19:00 and there will be
no service from 21:30 on either line.
From Saturday 27 until Monday 29 March, there will be no service on either
line.

From 18:00 on Saturday 27, there will be no service on the Metropolitan
line, and on Sunday 28 March, there will be no service.

In addition, the Piccadilly line between Rayners Lane and Uxbridge and the
District line between High Street Kensington and Edgware Road may also be
disrupted at these times.

The disruptions could also affect Chiltern Railway services.

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Just curious — is this industrial action, or a genuine staff shortage? Or
something to do with the new signalling?