A wiring error had caused the preceding signal to incorrectly display a green aspect.
The track circuits connected to the signal cabin at Charing Cross had been converted to alternating current on the morning of 8 May.
The cabin was put into automatic mode as this was how it was normally set, but no testing was carried out, and traffic began with no observation from the engineers.
The message was conveyed to Charing Cross but no action was taken despite further reports that the signal was working incorrectly.
A similar accident involving a signal wiring error had occurred near the same station on the Northern line on 10 March 1938.