The report found progress has slowed on Tube improvement projects
Tube trains could become more crowded and improvements on London Underground scrapped unless a "funding crisis" is resolved, the London Assembly has said.
Transport for London (TfL) and London Underground maintenance firm Tube Lines disagree on the cost of upgrade work.
Plans to tackle congestion and improve access to stations are being put at risk due to the disagreement, the London Assembly (LA) report found.
London Underground (LU) said it tried to make the work affordable.
Last year Tube Lines was told it could legitimately charge up to £5.5bn for improvement work it will carry out between 2010 and 2017.
LU had estimated the cost at £4.1bn but Tube Lines set its price at £7.2bn.
The LA report said the discrepancy between these figures had slowed progress on a number of projects and put pressure on TfL's budget.
The situation was exacerbated by the collapse of maintenance firm Metronet, which went into administration in 2007 while carrying out station refurbishments.
TfL is now completing Metronet's projects but around half its 150 refurbishment plans that were due to be completed by 2010 have been delayed until as late as 2017.
The LA called for TfL to revise its plans to take into account the increasing cost of upgrade works and called for it be more open and accountable about progress made.
LA transport committee chairwoman Val Shawcross said: "Maintaining and upgrading the Tube is absolutely vital to the capital and Londoners have a right to know how it is progressing."
Welcoming the report, LU managing director Tim O'Toole said: "LU has issued restated contract terms to Tube Lines for the second period of public private partnership (PPP) maintenance and renewal works.
"We have made every effort to make the works as affordable as possible. Tube Lines now has until the end of June 2009 to respond."
A Tube Lines spokesman said the report "rightly notes that we are a well-managed business that is successfully delivering vital improvements to the Tube".
He added: "There is still so much more to do on the Tube, particularly with regards to increasing capacity for passengers and we therefore hope that the funding that is needed to achieve this continues."
A Department for Transport spokesman said: "We have already agreed a generous long-term settlement with TfL, providing more than £40bn for London transport over the next 10 years.
"It is now for TfL to manage this to deliver the high quality transport its users expect."