Why Neville Chamberlain resigned but remained in government

Was Neville Chamberlain's resignation political or physical, and what if he had stayed longer?

It was primarily political: he had lost confidence and would have been forced out, though he remained an important figure in the coalition afterward.

Chamberlain's resignation is explained here as a response to lost political confidence. Remaining prime minister would, in this assessment, have led to his being forced out through Parliament. Resignation still left room for him to serve in the national coalition. His departure from the premiership therefore needs to be distinguished from an immediate departure from government, without making physical incapacity the principal explanation.