A series of Tube derailments have helped create a £55m shortfall in Transport for London's (TfL) budget.
A Northern Line train hit a wall when it derailed at Camden
The Chancery Lane derailment in January left the transport body facing a £50m fall in predicted income.
But October's Camden Town and Hammersmith derailments pushed this figure up by £5m, TfL's board heard.
It comes as a trackside meeting between the Tube's managing director and a union leader failed to stop planned industrial action over safety.
Jay Walder, TfL's finance director, said the summers' heatwave and improvements to buses may have resulted in people switching to buses, adding to the shortfall.
The 11-week shut down of the Central Line, as a result of the Chancery Lane crash, and on-going service and safety concerns account for half the drop in demand to use the Underground.
And it is estimated that 50,000 daily trips that were being made on the Tube have transferred to buses, resulting in a loss of Tube revenue of about £25m-per-year, according to TfL figures.
But bus revenues are up by £27m above budget and usage has increased overall by 10% this year.
Mr Walder said the predicted shortfall in Tube revenue for 2003/2004 is recognised in the revenue projections being made for the spending revue 2004.
As a result of the derailments the Rail, Maritime and Transport (RMT) union has planned a 48-hour "go-slow" on 9 and 10 December.
The union is campaigning for maintenance work to be taken away from private firms and given back to London Underground (LU).
The RMT wants maintenance taken away from private companies
It has also threatened further industrial action in the week before Christmas if its demands are not met.
In response, LU's managing director Tim O'Toole met Bob Crow, general secretary of the RMT, near Hammersmith - the scene of one of the derailments - at 0030 GMT on Wednesday.
Mr O'Toole said: "I am making every effort to demonstrate to the unions that I am listening to their safety concerns.
"A safe Tube is my top priority."
He said taking industrial action would do nothing to improve safety.
But Mr Crow, who welcomed Mr O'Toole comments, said the inspection of the track had shown that lighting and other standards were "unsatisfactory".
He added: "We need to see concrete changes, and next week's safety go-slow is still on."
The two men are due to meet on Thursday for further talks.