Paul Corfield wrote:

The Picc Line has one of the highest proportions of RMT drivers on the
network so it's no great surprise. If they were to run only on Tube
Lines maintained tracks that would mean Heathrow - Northfields and
Cockfosters - Hyde Park Corner only for the next three days. Even that
may be impossible because of maintenance and crewing issues as the stock
and crews would probably be imbalanced on the two isolated sections.

UPDATE at 20:22:

Piccadilly Line now suspended Hyde Park Corner to Northfields/Uxbridge.