Private firms are "failing miserably" to deliver promised improvements to the ageing Tube network, politicians say.
Tube passengers are still waiting for improvements, say the Lib Dems
Metronet in particular is significantly behind on promised track improvements, say Lib Dems on the London Assembly.
The Jubilee Line will shut at Christmas and there are proposals to shut parts of the Northern Line, the busiest line, to allow work to be carried out.
Metronet says while there have been delays to its programme, it is now bringing key improvements to the Tube.
Responsibility for maintaining and improving London's underground network passed to two infrastructure companies (infraco) in 2003.
Metronet and Tube Lines were given 30 year contracts worth £15bn under the controversial Public Private Partnership scheme (PPP).
But Lib Dem Assembly members say documents show Transport for London (TfL) does not believe they will be able to deliver what they promised.
Lib Dem transport spokesman Lynne Featherstone said: "Metronet in particular appear to have over-promised and overstretched themselves on what upgrades they could realistically deliver for passengers."
On Tuesday the mayor announced the Jubilee Line is to close from 25 to 30 December to allow for an extra carriage to trains - part of improvements pledged in the PPP contract.
Services will be scaled down in December to allow testing of an improved signalling system and the new seven-carriage trains.
Tube Lines has also requested that small sections of the Northern Line - the busiest but worst-performing line - be shut for weeks at a time to allow them to upgrade tracks "much faster".
London Underground managing director Tim O'Toole said they would look at the proposals, but would not take any decisions lightly.
Meanwhile Metronet has announced it is making 95 people redundant after it says it had "identified the optimum staff resource necessary for the efficient delivery of its multi-billion pound programme".
A spokesman added that Metronet was "delivering key improvements to the day-to-day operation of the Tube".